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

signed main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int n,d; cin >> n >> d;
  vector<int> t(n),k(n);
  for(int i = 0; i < n; i++){
    cin >> t[i] >> k[i];
  }

  vector<int> dp1(n+1); //tokyo
  vector<int> dp2(n+1); //kyoto
  dp1[0] = 0;
  dp2[0] = -d;
  for(int i = 0; i < n; i++){
    dp1[i+1] = max(dp1[i]+t[i],dp2[i]+t[i]-d);
    dp2[i+1] = max(dp2[i]+k[i],dp1[i]+k[i]-d);
  }
  cout << max(dp1[n],dp2[n]) << endl;

  return 0;
}