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: Wrong Answer)

#include<stdio.h>
#include<stdlib.h>
#define MAX 100000
typedef struct
{
	int A;
	int B;
}omise;

int cmp(const void *p, const void *q) {
	return ((omise*)p)->A - ((omise*)q)->A;
}


int main()
{
	int N, M,i;
	omise list[MAX];

	int kingaku = 0;

	scanf("%d %d", &N, &M);
	for (i = 0;i < N;i++)
	{
		scanf("%d %d", &list[i].A, &list[i].B);
	}
	qsort(list, N, sizeof(omise), cmp);
	for (i = 0;M>0;i++)
	{
		if(M>list[i].B)
		kingaku += (list[i].A*list[i].B);
		else kingaku += (list[i].A*M);
		M -= list[i].B;
	}
	printf("%d\n",kingaku);
	return 0;
}