差分+ 贪心

#include <iostream>
#include <vector>
using namespace std;

const int N = 100010;
int n, m, k;
vector<int> b(N); 
typedef long long LL;

int main(int argc, char *argv[]) {
  cin >> n >> m >> k;
  for (int i = 0; i < n; i++) {
    int t, c;
    cin >> t >> c;
    b[1] += c, b[t + 1] -= c;
  }
  for (int i = 1; i < N; i++) {
    b[i] += b[i - 1];
  }

  LL costs{0};
  int i = N - 1;
  while (i > k && costs + b[i] <= m) costs += b[i--];
  cout << i << endl;
  return 0;
}