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: Time Limit Exceeded)

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

    var N = parseInt(one[0], 10);
    var M = parseInt(one[1], 10);
    var purchase_count = 0;
    var purchase_money = 0;

    // 店情報
    var shop_info = {
        Money: '',
        Max_count: ''
    };

    var shops_info = [];

    for (var i = 0; i < N; i++) {
        Drinks = args[i + 1].split(' ');
        shops_info[i] = { Money: parseInt(Drinks[0], 10), Max_count: parseInt(Drinks[1], 10)}
    }

    shops_info.sort((a, b) => { return (a.Money < b.Money) ? -1 : 1;})

    var target_shop;

    for (var i = 0; i < N; i++) {
        target_shop = shops_info[i]
        if(target_shop.Max_count + purchase_count <= M) {
            purchase_money += target_shop.Max_count * target_shop.Money;
            purchase_count += target_shop.Max_count;
        } else {
            target_count = M - purchase_count;
            purchase_money += target_shop.Money * target_count;
            break;
            }
        }

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