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 (Clang 3.8.0) (judge verdict: Accepted)

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

typedef struct
{
	long	price;
	int		num;
} t_shop;


int	cmp(const void *a, const void *b)
{
	t_shop *s1 = (t_shop *)a;
	t_shop *s2 = (t_shop *)b;

	if (s1->price < s2->price)
	{
		return (-1);
	}
	if (s1->price > s2->price)
	{
		return (1);
	}
	return (0);
}

int	main(void)
{
	int		n;
	int		m;
	t_shop	shops[100000];
	long	money = 0;

	scanf("%d %d", &n, &m);
	for (int i = 0; i < n; i++)
	{
		scanf("%ld %d", &shops[i].price, &shops[i].num);
	}
	qsort(shops, n, sizeof(t_shop), cmp);
	for (int i = 0; i < n; i++)
	{
		if (shops[i].num >= m)
		{
			printf("%ld\n", money + shops[i].price * m);
			return (0);
		}
		else
		{
			money += shops[i].price * shops[i].num;
			m -= shops[i].num;
		}
	}
	return (0);
}