#include <bits/stdc++.h>

using namespace std;

#define int long long

signed main() {
  int n, d;
  cin >> n >> d;
  vector<int> t(n), k(n);
  for(int i = 0; i < n; ++i) cin >> t[i] >> k[i];
  vector<vector<int> > dp(2, vector<int>(n+1, -1));
  dp[0][0] = 0;
  dp[1][0] = -(1LL<<55);
  for(int i = 0; i < n; ++i) {
    dp[0][i+1] = max(dp[0][i]+t[i], dp[1][i]+t[i]-d);
    dp[1][i+1] = max(dp[0][i]+k[i]-d, dp[1][i]+k[i]);
  }
  cout << max(dp[0][n], dp[1][n]) << endl;

  return 0;
}