#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

int main(){
  int N, D; cin >> N >> D;
  int tokyo = 0, kyoto = 0;
  for(int i = 0; i < N; i++){
    int T, K; cin >> T >> K;
    if(i == 0){
      K = max(K-D, 0);
    }
    int tokyo2 = max(tokyo + T, kyoto + T - D);
    int kyoto2 = max(tokyo + K - D, kyoto + K);
    tokyo = tokyo2, kyoto = kyoto2;
    // cout << tokyo << " " << kyoto << endl;
  }
  cout << max(tokyo, kyoto) << endl;
}