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

int N, D;
int T[100], K[100];

int main() {
  ios::sync_with_stdio(false);
  cin >> N >> D;
  for (int i = 0; i < N; ++i) {
    cin >> T[i] >> K[i];
  }
  int a = 0, b = 0xc0c0c0c0;
  for (int i = 0; i < N; ++i) {
    int na = max(a, b - D) + T[i];
    int nb = max(b, a - D) + K[i];
    a = na;
    b = nb;
  }
  cout << max(a, b) << endl;
  return 0;
}