#include<bits/stdc++.h>
#define setIO(s) freopen(s".in","r",stdin) 
#define ll long long   
using namespace std;
int main()
{ 
	int n; 
	ll s,c,y,cur=0,ans=0;   
	scanf("%d%lld",&n,&s); 
	scanf("%lld%lld",&c,&y); 
	ans+=c*y, cur=c; 
	for(int i=2;i<=n;++i)
	{
		scanf("%lld%lld",&c,&y); 
		cur=min(cur+s,c); 
		ans+=cur*y; 
	}
	printf("%lld\n",ans); 
	return 0; 
}