#include <bits/stdc++.h>

using namespace std;

int main(void) {
  int n, d;
  cin >> n >> d;

  vector<int> t(n), k(n);
  for (int i = 0; i < n; i++) {
    cin >> t[i] >> k[i];
  }

  int dp[110][2] = {};
  dp[0][1] = -d;
  for (int i = 0; i < n; i++) {
    dp[i + 1][0] = max(dp[i][0] + t[i], dp[i][1] + k[i] - d);
    dp[i + 1][1] = max(dp[i][0] + t[i] - d, dp[i][1] + k[i]);
  }
  cout << max(dp[n][0], dp[n][1]) << endl;

  return 0;
}