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 = (sdi) => {
  const line = sdi.split("\n");
  const temp = line.shift().split(" ");
  const N = temp[0]-0;
  let M = temp[1]-0;
  let a = new Array(line.length);
  for(const j in line){
    a[j] = line[j].split(" ").map( str => parseInt(str, 10) );
  }
  a.sort((a,b) => {
    return(a[0] - b[0]);
  });
  let money = 0;
  let buy = 0;
  for(const i in a){
    if(M === 0) break;
    buy = Math.min(a[i][1], M);
    money = money + (a[i][0] * buy);
    M = M - buy;
  }
  console.log(money);
}

main(require('fs').readFileSync('/dev/stdin', 'UTF-8'));