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 JavaScript (node.js v5.12) (judge verdict: Runtime Error)

'use strict'

function main(input) {
    const args = input.split('\n');
    const oneL = args[0].split(' ');

    const needDrinkNum = parseInt(oneL[1], 10);

    let shopProp = args.slice(1).map(l1 => {
        return l1.split(' ').map(l2 => parseInt(l2, 10));
    });

    shopProp.sort((a, b) => {
        if (a[0] < b[0]) {
            return -1;
        }
        if (a[0] > b[0]) {
            return 1;
        }
        return 0;
    });

    let value = 0;

    for (let i = 0; i !== needDrinkNum; i++) {
        while (shopProps[i][1] !== 0) {
            value += shopProps[i][0];
            shopProps[i][1] = shopProps[i][1] - 1;
        }
    }

    console.log(value);
}
 
main(require('fs').readFileSync('/dev/stdin', 'utf8'));