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: Time Limit Exceeded)

function compare(a,b){
  if(a.cost > b.cost){
    return 1
  }else{
    return -1
  }
}

// inputに入力データ全体が入る
function Main(input) {
	// 1行目がinput[0], 2行目がinput[1], …に入る
  input = input.split("\n")
  tmp0 = input[0].split(" ")
  N = Number(tmp0[0])
  M = Number(tmp0[1])

  var ans = 0;
  info = []
  for(var i = 0; i < N; i++)
  {
    var tmp = input[i+1].split(" ")
    info.push({cost: Number(tmp[0]), can: Number(tmp[1])})
  }

  info.sort(compare)
  var buy_cans = 0

  for(var i=0; i < N; i++)
  {
    var need_cans = M - buy_cans
    if((need_cans - info[i].can) <= 0){
      ans += info[i].cost * need_cans
      console.log(ans)
      return
    }else{
      buy_cans += info[i].can
      ans += info[i].cost * info[i].can
    }
  }
}
//*この行以降は編集しないでください（標準入出力から一度に読み込み、Mainを呼び出します）
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
// Main("4 30\n6 18\n2 5\n3 10\n7 9\n")