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: Runtime Error)

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


int main() {
	int i, j, k, f = 0, n, s, t, m;
	int a, b, c, d, e;
	long long int x, y, z;
	int aa[1100000];
	int bb[1100000];
	int cc[1100000];

	scanf("%d %d", &n, &m);
	for (i = 0;i < n;i++) {
		scanf("%d %d", &aa[i], &bb[i]);
		cc[i] = aa[i] * bb[i];
	}

	x = 0;
	y = 0;
	e = 1100000000;

	for (i = 0;y < m;i++) {
		for (j = 0;j < n;j++) {
			if (e > aa[i]) {
				e = aa[i];
				t = i;
			}
		}
		if (y + bb[t] > m) {
			x = x + (m - y)*aa[t];
			break;
		}
		x = x + (bb[t] * aa[t]);
		aa[t] = 9999999999;
	}




	printf("%lld", x);

	return 0;
}