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{
    long long price;
    long long num;
} store;

int sort(const void *a, const void *b)
{
    store *A = (store *)a;
    store *B = (store *)b;
    return A->price - B->price;
}

int main(void)
{
    long long i, n, m, sum = 0;
    scanf("%lld%lld", &n, &m);
    store AB[n];

    for ( i = 0; i < n; i++)
        scanf("%lld%lld", &AB[i].price, &AB[i].num);

    qsort(AB, n, sizeof(AB[0]), sort);

    i = 0;
    while(m > 0){
        if(m <= AB[i].num){
            sum += m * AB[i].price;
            break;
        }else{
            m -= AB[i].num;
            sum += AB[i].price * AB[i].num;
        }
        i++;
    }
    printf("%lld\n", sum);
    return 0;
}