#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  int res = INT_MIN;
  for (int i = 0; i < n; i++) {
    int a, b;
    cin >> a >> b;
    res = max(res, (a + min(0, (k - b))));
  }
  cout << res << endl;
  return 0;
}
