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>
int cmpfunc (const void * a, const void * b)
{
    return ( ((long long int*)a)[0] - ((long long int*)b)[0] );
}
int main(void){
    
    int n=0,i;
    long long int money=0,m=0;
    scanf("%d %lld",&n,&m);
    long long int a[n][2];
    
    for(i=0;i<n;i++){
        scanf("%lld %lld",&a[i][0],&a[i][1]); // change
        //a[i][1] = i; // follow
    }
    
    qsort(a,n,sizeof(long long int)*2,cmpfunc);
    
    for(int i=0;i<n;i++){
        if(m-a[i][1]>0){
            money = money + a[i][0]*a[i][1];
            m = m-a[i][1];
        }else{
            money = money + m*a[i][0];
            printf("%lld\n",money);
        }
    }
  
    
    
    system("pause");
    return 0;
    
}