#include<bits/stdc++.h>
using namespace std;
unsigned long long N,S,c,y,minn=0x7f7f7f7f,sum=0;
int main(){
	freopen("./P1376_6in.txt","r",stdin);
	scanf("%d%d",&N,&S);
	for(int i=0;i<N;i++){
		scanf("%d%d",&c,&y);
		minn=min(minn,c);
		sum+=minn*y;
		minn+=S;	
	}
	printf("%d",sum);
	return 0;
}