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 Drink{
	int A;
	int B;
};

int desc(const void *a, const void *b){
	struct Drink *A = (struct Drink *)a;
	struct Drink *B = (struct Drink *)b;
	return A->A - B->A;

}

int main(){
	int N, M, *A, *B, i, j, count = 0, sum = 0;

	struct Drink *d;

	scanf("%d%d", &N, &M);
	d = (struct Drink *)malloc(sizeof(struct Drink)*N);
	for(i = 0; i < N; i++){
		scanf("%d%d", &(d[i].A), &(d[i].B));
	}

	qsort(d, N, sizeof(struct Drink), desc);
	
	for(i = 0; i < N; i++){
		for(j = 0; j < d[i].B; j++){
			sum += d[i].A;
			count++;
			if(count == M){ break; }
		}
		if(count == M){ break; }
	}

	printf("%d", sum);

	return 0;
}