#include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
int n,s,c,a,i;
long long now=10000000,ans=0;
int main()
{
   	freopen("factory.in","r",stdin);
   	freopen("factory.out","w",stdout);
    scanf("%d%d",&n,&s);//n个月，保管费用为s
    for (i=1;i<=n;i++)
    {
        scanf("%d%d",&c,&a);//当月的成本为c,需求量为a
        if(now>c) //如果从[1,i-1]这一段某个月生产的成本高于本月的
            now=c;//本月自己生产奶酪
        ans+=now*a;
        now+=s;//累加储存成本
    }
    cout<<ans<<endl;
    return 0;
}