#include <iostream>
using namespace std;
typedef long long LL;
int main(int argc, char* argv[])
{
	int N;
	LL d,t,k;
	cin>>N>>d;
	LL T=0,K=-d;//i-1日の最大利益
	int i;
	LL Sum=0;
	for (i=0;i<N;i++){
		cin>>t>>k;
		LL t1=max(T,K-d)+t;
		LL k1=max(T-d,K)+k;
		T=t1;
		K=k1;
	}
	cout<<max(T,K)<<endl;
	return 0;
}