#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main(void){
  int n, d;
  cin >> n >> d;
  int t[n + 1], k[n + 1];
  for(int i = 1; i <= n; i++) cin >> t[i] >> k[i];
  long long ans[2][n + 1];
  ans[0][0] = 0;
  ans[1][0] = -d;
  for(int i = 1; i <= n; i++){
    ans[0][i] = max(ans[0][i - 1] + t[i], ans[1][i - 1] - d + t[i]);
    ans[1][i] = max(ans[1][i - 1] + k[i], ans[0][i - 1] - d + k[i]);
  }
  cout << max(ans[0][n], ans[1][n]) << endl;
  return 0;
}