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>

struct input{
   long int a,b;
};

int compare_int(const void *x,const void *y){
    if ( (*(struct input*)x).a > (*(struct input*)y).a) return 1;
    if ( (*(struct input*)x).a < (*(struct input*)y).a) return -1;
    return 0;
}

int main(void){
    long int n,m,k,ans,i,j;
    k = 0, ans = 0;
    scanf("%d%d",&n,&m);
    struct input A[n];
    for(i=0; i<n; i++){
        scanf("%ld%ld",&(A[i].a),&(A[i].b) );
    }
    qsort(A, n, sizeof(struct input), compare_int);


    for(i=0; k<=m; i++){
        if( k + A[i].b >= m ){
            ans += A[i].a * (m - k);
            k += m - k;
            break;
        }else{
            ans += A[i].a * A[i].b;
            k += A[i].b;
        }
    }
    printf("%ld\n",ans);

    return 0;
}