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>

#define DEBUG 1
#define NINF -1
#define PINF 1E9+1

typedef struct {
	int a;
	int b;
} Money;

int n,m;
Money *money;

int compareMoney(const void *sub1,const void *sub2)
{
	return ((Money *)sub1)->a - ((Money *)sub2)->a;
}

int main()
{
	long ret;
	int i,j;

	ret = 0;

	scanf("%d %d",&n,&m);
	money = (Money *)malloc(sizeof(Money) * n);
	for (i=0;i<n;i++) {
		scanf("%d %d",&money[i].a,&money[i].b);
	}
	
	qsort(money,n,sizeof(Money),compareMoney);

	for (i=0;i<n;i++) {
		if (m <= money[i].b) {
			ret += (long)money[i].a * m;
			break;
		}

		ret += (long)money[i].a * money[i].b;
		m -= money[i].b;
	}

	printf("%ld\n",ret);
	free(money);
	return 0;
}