#include <bits/stdc++.h>

using namespace std;

int n,m,ans;

struct milk
{
    int a,b;
}a[5005];

bool cmp(milk a,milk b)
{
    if(a.a != b.a)
        return a.a < b.a;
    return a.b > b.b;
}

int main()
{
    scanf("%d%d",&n,&m);
    for(int i = 1;i <= n;i++)
        scanf("%d%d",&a[i].a,&a[i].b);
    sort(a + 1,a + n + 1,cmp);
    int i = 1;
    while(m)
    {
        if(a[i].b)
        {
            a[i].b--;
            ans += a[i].a;
            n--;
        }
        else
            i++;
    }
    printf("%d",ans);
    return 0;
}

RE,蒟蒻求助