#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
int c,s;
int main()
{
    int n,m;
    scanf("%d%d",&n,&m);
    long long ans=0;
    scanf("%d%d",&c,&s);
    int k=c;
    ans+=k*s;
    for(int i=1;i<n;i++)
    {
        scanf("%d%d",&c,&s);
        k=min(k+m,c);
        ans+=k*s;
    }
    printf("%lld\n",ans);
    return 0;
}