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: Wrong Answer)

"use strict";

function Main(input) {
    input = input.split("\n");

    input[0] = input[0].split(" ");
    const n = parseInt(input[0][0], 10);
    const m = parseInt(input[0][1], 10);

    let stores = input.slice(1).map(x => x.split(" ").map(x => parseInt(x, 10)));

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

    let numDrink = 0;
    let totalPrice = 0;
    for (let i = 0; i < n; i++) {
        const store = stores[i];
        for (let j = 0; j < store[1]; j++) {
            if (numDrink === m) {
                console.log(totalPrice);
                return;
            }

            numDrink++;
            totalPrice += store[0];
        }
    }
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));