Problem:
Score : 300 points

Problem Statement

Hearing that energy drinks increase rating in those sites, Takahashi decides to buy up M cans of energy drinks.

There are N stores that sell energy drinks. In the i-th store, he can buy at most B_i cans of energy drinks for A_i yen (the currency of Japan) each.

What is the minimum amount of money with which he can buy M cans of energy drinks?

It is guaranteed that, in the given inputs, a sufficient amount of money can always buy M cans of energy drinks.

Constraints

All values in input are integers.

1 \leq N, M \leq 10^5

1 \leq A_i \leq 10^9

1 \leq B_i \leq 10^5

B_1 + ... + B_N \geq M

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
\vdots
A_N B_N

Output

Print the minimum amount of money with which Takahashi can buy M cans of energy drinks.

Sample Input 1

2 5
4 9
2 4

Sample Output 1

12

With 12 yen, we can buy one drink at the first store and four drinks at the second store, for the total of five drinks. However, we cannot buy 5 drinks with 11 yen or less.

Sample Input 2

4 30
6 18
2 5
3 10
7 9

Sample Output 2

130

Sample Input 3

1 100000
1000000000 100000

Sample Output 3

100000000000000

The output may not fit into a 32-bit integer type.

Solution in C (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>

typedef struct{
    long long int a;
    int b;
} drink;

int compare_int(const void *a,const void *b){
    return *(int*)a-*(int*)b;
}


int main(){
    int n,m;
    int i;
    long long int ans;
    scanf("%d %d",&n,&m);
    drink s[n];
    for(scanf("%lld %d",&s[i].a,&s[i].b);i<n;i++);
    qsort(s,n,sizeof(drink),compare_int);
    i=0;
    while(m>s[i].b&&m>0&&i<n){
        ans+=s[i].b*s[i].a;
        m-=s[i].b;
        i++;
    }
    ans+=m*s[i].a;
    printf("%lld",ans);
    return 0;
}