#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n, k;
  cin >> n >> k;
  long long ans = INT_MIN;
  for (long long i = 0; i < n; i++) {
    long long temp, j, t;
    cin >> j >> t;
    if (t > k) {
      temp = j - (t - k);
    } else
      temp = j;
    ans = max(temp, ans);
  }
  cout << ans;
  return 0;
}
