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";
    
const main = arg => {
    const N = parseInt(arg.split("\n")[0].split(" ")[0]);
    const M = parseInt(arg.split("\n")[0].split(" ")[1]);
    const temp = arg.split("\n").slice(1);
    
    let obj = [];
    
    for(let i in temp) {
        obj[i] = {};
        obj[i].price = temp[i].split(" ")[0];
        obj[i].stock = temp[i].split(" ")[1];
    }
    
    obj.sort((a,b)=>(a.price - b.price));
    
    let done = 0;
    let need = M;
    let ans  = 0;
    
    for(let i in obj) {
        if(obj[i].stock < need) {
            ans += parseInt(obj[i].price * obj[i].stock);
            done += parseInt(obj[i].stock);
            need -= parseInt(obj[i].stock);
        } else {
            ans += parseInt(obj[i].price * need);
            done += parseInt(need);
            console.log(parseInt(ans));
        }
    }
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));