#include <bits/stdc++.h>

using namespace std;

int n,d;
int t[101],k[101];
int dp[105][2];
int main() {
  cin >> n >> d;
  for(int i = 0; i < n; i++) cin >> t[i] >> k[i];

  dp[0][1] = -1e9;
  for(int i = 0; i < n; i++) {
    for(int j = 0; j < 2; j++) {
      if(j & 1) {
	dp[i+1][j] = max(dp[i+1][j],max(dp[i][j-1]+k[i]-d,dp[i][j]+k[i]));
	//	cout << dp[i+1][j] << endl;
      }
      else {
	dp[i+1][j] = max(dp[i+1][j],max(dp[i][j+1]+t[i]-d,dp[i][j]+t[i]));
	//	cout << dp[i+1][j] << endl;
      }
    }
  }

  cout << max(dp[n][0],dp[n][1]) << endl;
}