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)

function Main(input){
  input = input.split("\n");
  inputStr = input[0].split(" ");
  var N = Number.parseInt(inputStr[0]);
  var M = Number.parseInt(inputStr[1]);
  var tmp = [];
  var array = [];
  var buycnt = 0;
  var okane = 0;
  var mokori = M;
  //配列に数値化して溜め込む
  for (i=0;i<N;i++){
    tmp = input[i+1].split(" ");
    array[i] = tmp.map( tmp => parseInt(tmp,10) )
  }
  //ソートする
  array.sort();
  //必要本数を買い込む
  for (i=0 ; i<N ; i++){
	if (mokori >= array[i][1]){
      okane += array[i][0] * array[i][1];
      mokori -= array[i][1];
    } else {
      okane += array[i][0] * mokori;
      break;
    }
  }
  console.log(okane);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));