二分 + 贪心

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

const int N = 100010;
int n, m, k;
vector<int> t(N);
vector<int> c(N);

typedef long long LL;

bool Check(const int T) {
  LL costs{0};
  for (int i = 0; i < n; i++) {
    if (t[i] > T) {
      int delta = t[i] - T;
      costs += 1LL * delta * c[i];
      if (costs > m) return false;
    }
  }
  return costs <= m;
}

int main(int argc, char *argv[]) {
  cin >> n >> m >> k;
  for (int i = 0; i < n; i++) {
    cin >> t[i] >> c[i];
  }
  int l = k, r = *max_element(t.begin(), t.begin() + n);
  int res{r};
  while (l <= r) {
    int mid = l + (r - l) / 2;
    if (Check(mid)) {
      res = mid;
      r = mid - 1;
    } else {
      l = mid + 1;
    }
  }
  cout << res << endl;
  return 0;
}