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: Time Limit Exceeded)

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

void    sort(int **ab, int n)
{
	int *temp;
	for (int i = 0; i < n - 1; i++)
	{
		for (int i = 0; i < n - 1; i++)
		{
			if (ab[i][0] > ab[i + 1][0])
			{
				temp = ab[i];
				ab[i] = ab[i + 1];
				ab[i + 1] = temp;
			}
		}
	}
}

int		main(void)
{
	int		n,m;
	scanf("%d %d", &n, &m);
	int		**ab = (int **)malloc(sizeof(int *) * n);
	for (int i = 0; i < n; i++)
		ab[i] = (int *)malloc(sizeof(int) * 2);
	for (int i = 0; i < n; i++)
		scanf("%d %d", &ab[i][0], &ab[i][1]);
	sort(ab, n);
	int		count = 0;
	long	sum = 0;
	for (int i = 0; i < n; i++)
	{
		for (int j = 0; count < m && j < ab[i][1]; j++)
		{
			sum += ab[i][0];
			count++;
		}
	}
	printf("%ld\n", sum);

}