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: Runtime Error)

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

typedef struct{
    long int A;
    long int B;
}imput;

int cmp( const void *p, const void *q ) { 
    long int *A1 = ((imput*)p)->A;
    long int *A2 = ((imput*)q)->A;
    if(*A1 > *A2) return 1;
    if(*A1 > *A2) return -1;
    return 0;
}

int main(){
    imput AB[11000];
    long int N, M;
    long int tmp, Mtmp, ans=0;
    int i, j;

    scanf("%d %d", &N, &M);

    for(i=0; i<N; i++){
        scanf("%ld %ld", &AB[i].A, &AB[i].B);
    }

    qsort(AB, N, sizeof(AB), cmp);

    Mtmp = M;

    for(i=0; i<N; i++){
        Mtmp = Mtmp - AB[i].B;
        if(Mtmp >= 0){
            ans += AB[i].A*AB[i].B;
        }else{
            ans += AB[i].A*(Mtmp+AB[i].B);
            break;
        }
    }

    printf("%ld", ans);

    return 0;

}