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#include<bits/stdc++.h>
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const int N = 5e5 + 10;
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using namespace std;
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#define ll long long
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int n, m;
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struct U
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{
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  ll l, r;
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  ll length;
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  bool operator < (U o) const
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  {
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    return length < o.length;
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  }
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} segment[N];
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ll b[N << 1], btop;
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ll c[N << 1], ctop;
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struct Seg
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{
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  #define ls (u << 1)
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  #define rs ((u << 1) | 1)
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  int tree[N << 4];
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  int tag[N << 4];
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  void pushup(int u)
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  {
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    tree[u] = max(tree[ls], tree[rs]);
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  }
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  void pushdown(int u, int l, int r)
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  {
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    int mid = (l + r) >> 1;
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    tree[ls] +=  tag[u];
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    tree[rs] +=  tag[u];
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    tag[ls] += tag[u], tag[rs] += tag[u];
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    tag[u] = 0;
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  }
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  void change(int u, int l, int r, int L, int R, int val)
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  {
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    if(L <= l && r <= R)
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    {
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      tree[u] += val;
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      tag[u] += val;
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      return;
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    }
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    pushdown(u, l, r);
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    int mid = (l + r) >> 1;
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    if(L <= mid) change(ls, l, mid, L, R, val);
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    if(R > mid) change(rs, mid + 1, r, L, R, val);
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    pushup(u);
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  }
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  int query()
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  {
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//    if(L <= l && r <= R)
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//    {
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//      return tree[u];
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//    }
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//    pushdown(u, l, r);
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//    int mid = (l + r) >> 1;
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//    ans = 0;
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//    if(L <= mid) ans = max(ans, query(ls, l, mid, L, R));
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//    if(R > mid) ans = max(ans, query(rs, mid + 1, r, L, R));
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//    return ans;
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// complex!!!
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    return tree[1];
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  }
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}sgtree;
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int main()
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{
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//  cout << "qwq" << sgtree.tree[10];
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  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
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  cin >> n >> m;
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  for(int i = 1;i <= n;i++)
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  {
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    cin >> segment[i].l >> segment[i].r;
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    segment[i].length = segment[i].r - segment[i].l;
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    b[++btop] = segment[i].l, b[++btop] = segment[i].r;
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  }
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  b[0] = -1;
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  sort(b + 1, b + 1 + btop);
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  for(int i = 1;i <= btop;i++)
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  {
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    if(b[i] != b[i - 1])
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      c[++ctop] = b[i];
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  }
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  for(int i = 1;i <= n;i++)
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  {
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    segment[i].l = lower_bound(c + 1, c + 1 + ctop, segment[i].l) - c;
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    segment[i].r = lower_bound(c + 1, c + 1 + ctop, segment[i].r) - c;
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  }
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  sort(segment + 1, segment + 1 + n);
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  int l = 1, r = 1;
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  ll ans = 1145141919810;
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  for(int r = 1;r <= n;r++)
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  {
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//    cout << r << "qwq"<<endl;
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    sgtree.change(1, 1, ctop, segment[r].l, segment[r].r, 1);
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//    cout << r <<" -- " << sgtree.tree[1] << endl;
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    while(sgtree.query() >= m)
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    {
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      ans = min(ans, segment[r].length - segment[l].length);
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      sgtree.change(1, 1, ctop, segment[l].l, segment[l].r, -1);
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      l++;
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    }
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  }
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  if(ans == 1145141919810)
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  {
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    printf("-1");
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    return 0;
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  }
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  else
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  {
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    cout << ans << endl;
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  }
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  return 0;
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}