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>
#include <string.h>

struct dorinku {
	long long int A;
	int B;
};

int asc(const void *a, const void *b) {
	struct dorinku *x = (struct dorinku *)a;
	struct dorinku *y = (struct dorinku *)b;
	if (x->A > y->A) return 1;
	if (x->A < y->A) return -1;
	return 0;
}

int main() {
	int N, M, i;
	long long int sum = 0;
	struct dorinku AB[100000];

	scanf("%d %d", &N, &M);
	for (i = 0; i < N; i++) {
		scanf("%lld %d", &AB[i].A, &AB[i].B);
	}

	qsort(AB, N, sizeof(struct dorinku), asc);

	for (i = 0; M > 0; i++) {
		if (M >= AB[i].B) {
			M = M - AB[i].B;
			sum = sum + AB[i].A * AB[i].B;
		}
		else {
			sum = sum + M * AB[i].A;
			M = 0;
		}
	}

	printf("%lld", sum);

	return 0;
}