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: Accepted)

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

typedef struct price_amount{
    int price;
    int amount;
}p_a;

int mycompare(const void * a, const void * b)
{
    return ( ((p_a*)a)->price - ((p_a*)b)->price );
}

int main() {
    int N,M;
    int i;
    
    scanf("%d %d", &N, &M);
    
    p_a *pa_list = malloc(sizeof(p_a)*N);
    
    for (i=0; i<N; i++){
        scanf("%d %d", &pa_list[i].price, &pa_list[i].amount);        
    }
    
    qsort(pa_list, N, sizeof(p_a), mycompare);
    
    unsigned long long int ans = 0;
    for (i=0; M>0; i++){
        if (pa_list[i].amount >= M){
            ans += (unsigned long long)pa_list[i].price * (unsigned long long)M;
            M = 0;
            break;
        } else {
            ans += (unsigned long long)pa_list[i].price * (unsigned long long)pa_list[i].amount;
            M -= pa_list[i].amount;
        }
    }
    /*
    printf("\n\n");
    for (i=0; i<N; i++){
        printf("%d %d\n", pa_list[i].price, pa_list[i].amount);
    }
    */
    printf("%llu", ans);
    return 0;
}