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>

struct K{
	long long A;
	long long B;
};

int asc(const void *a, const void *b) {
	struct K *X = (struct K *)a;
	struct K *Y = (struct K *)b;
	return X->A - Y->A;
}

int main() {
	struct K Ks[1000];
	long long i, N, M, cko = 0, csu = 0;

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

	for (i = 0; i < N; i++) scanf("%lld %lld", &Ks[i].A, &Ks[i].B);

	qsort(Ks, N, sizeof(struct K), asc);

	for (i = 0; i < N; i++) {
		if (cko + Ks[i].B < M) {
			cko += Ks[i].B;
			csu = csu + (Ks[i].A * Ks[i].B);
		}
		else {
			csu = csu + (Ks[i].A * (M - cko));
			break;
		}
	}

	printf("%lld\n", csu);

	return 0;
}