#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)

int n;
int d;
int x[100], y[100];

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  cin >> n >> d;
  REP (i, n) cin >> x[i] >> y[i];
  
  int t = 0, k = -1e9;

  REP (i, n) {
    int nt = max(t, k - d) + x[i];
    int nk = max(t - d, k) + y[i];
    t = nt;
    k = nk;
  }

  cout << max(t, k) << endl;
  return 0;
}