#include <bits/stdc++.h>
using namespace std;

int main() {
  int n, d;
  cin>>n>>d;
  int t[n], k[n];
  for (int i=0; i<n; i++) {
    cin>>t[i]>>k[i];
  }
  long long dp[n][2];
  for (int i=0; i<n; i++) dp[i][0] = dp[i][1] = 0;

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