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>

typedef struct{
	long a, b;
}store;

int comp(const void *a, const void *b){
	return (*(store*)a).a > (*(store*)b).a;
}

int main(void){
	int n, m;
	store *st;
	long sum = 0;
	scanf("%d %d", &n, &m);
	st = malloc(sizeof(store) * n);
	for(int i = 0; i < n; i++)
		scanf("%ld %d", &((st + i)->a), &((st + i)->b));
	qsort(st, n, sizeof(store), comp);
	int i = 0;
	while(m){
		if(st[i].b <= m){
			sum += st[i].a * (long)st[i].b;
			m -= st[i].b;
		//printf("%d:%d\n", st[i].a, st[i].b);
		}else{
			sum += st[i].a * m;
		//printf("%d:%d\n", st[i].a, m);
			m = 0;
		}
		i++;
	}
	printf("%ld\n", sum);
	return 0;
}