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: Accepted)

const main = (input) => {
  const inputs = input.trim().split('\n');
  const N = Number.parseInt(inputs[0].split(" ")[0]);
  var M = Number.parseInt(inputs[0].split(" ")[1]);

  var shops = [];
  for(var i=1;i<=N;i++){
    var obj = {
      pri: Number(inputs[i].split(" ")[0]),
      num: Number(inputs[i].split(" ")[1]),
    };
    shops.push(obj);
  }
  shops = shops.sort(function(a,b){return a.pri-b.pri});
  var ans = 0;
  for(var i=0;M>0;i++){
    var num = Math.min(M, shops[i].num);
    ans += num*shops[i].pri;
    M -= num;
  }
  console.log(ans);
}

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