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>

int main()
{
	int N,M;

	scanf("%d%d",&N,&M);


	int i,j;
	int A[N];
	int B[N];

	for(i=0; i<N; i++){
		scanf("%d%d", &A[i], &B[i]);
	}


	// ソート
	int temp1,temp2;

	for (i = 0 ; i < N ; i++){
		for ( j = i+1 ; j < N; j++){
			if (A[i] > A[j]){
				temp1 = A[j];
				A[j] = A[i];
				A[i] = temp1;
				temp2 = B[j];
				B[j] = B[i];
				B[i] = temp2;
			}
		}
	}


//そーとをしているかチェック
/*
	for (i = 0 ; i < N ; i++){
		printf("%d %d\n", A[i],B[i]);
	}
*/


	//購入
	long int tot=0;
	int count=M;

	for(i = 0 ; i < N ; i++){
		if (B[i]<=count){
			tot += (long int) A[i]*B[i];
			count = count - B[i];
		} else {
			tot += (long int) A[i]*(count);
			count = 0;
		}

		if (count == 0){break;}
	}


	printf("%ld\n", tot);

	return 0;
}