#include <iostream>
#include <algorithm>
#include <vector>
#include <string>

using namespace std;

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

  int dpL = 0;
  int dpR = -1.01e9;

  for (int i = 0; i < n; i++) {
    int l, r;
    cin >> l >> r;

    int tmpL = dpL;
    int tmpR = dpR;

    dpL = max(tmpL + l, tmpR + l - d);
    dpR = max(tmpL - d + r, tmpR + r);
  }

  cout << max(dpL, dpR) << endl;
}