#include<bits/stdc++.h>
using namespace std;
int n,s;
int c[10005],y[10005];
int sum;
bool kaiguan;
int main(){
	cin>>n>>s;
	for(int i=1;i<=n;i++){
		cin>>c[i]>>y[i];
	}
	for(int i=1;i<=n;i++){
		if(kaiguan!=true){
			sum+=y[i]*c[i];
		}
		else kaiguan=false;
		if(y[i+1]*s<(c[i+1]-c[i])*y[i+1]){
			sum+=c[i]*y[i+1];
			kaiguan=true;
			sum+=y[i+1]*s;
		}
		else continue;
	}
	cout<<sum;
	return 0;
}