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>

int main() {
    int N, M, X[10 ^5][2], i, j, tmp[2], num_item = 0, charge = 0;

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

    for (i = 0; i < N; i++) {
        scanf("%d%d", &X[i][0], &X[i][1]);
    }

    for (i = 0; i < N; ++i) {
        for (j = i + 1; j < N; ++j) {
            if (X[i][0] > X[j][0]) {
                tmp[0] = X[i][0];
                tmp[1] = X[i][1];
                X[i][0] = X[j][0];
                X[i][1] = X[j][1];
                X[j][0] = tmp[0];
                X[j][1] = tmp[1];
            }
        }
    }

    for (i = 0; i < N; i++) {
        while (X[i][1] > 0) {
            charge += X[i][0];
            X[i][1]--;
            num_item++;
            if (num_item == M) {
                break;
            }
        }
        if (num_item == M) {
            break;
        }
    }

    printf("%d", charge);

    return 0;
}