//スマホコーディング
#include<iostream>
using namespace std;

int tokyo[101] = {};
int kyoto[101] = {};

int main(){
int n, d, t, k;
cin >> n >> d >> t >> k;
tokyo[1] = t;
kyoto[1] = k-d;
for(int i = 2; i <= n; i++){
cin>>t>>k;
tokyo[i] = max(tokyo[i-1]+t, kyoto[i-1]+t-d);
kyoto[i] = max(tokyo[i-1]+k-d, kyoto[i-1]+k);
}

cout << max(tokyo[n],kyoto[n]) << endl;
return 0;
}