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 Scala (2.11.7) (judge verdict: Accepted)

import scala.io.{StdIn => in}

object Main extends App {
  val Array(n, m) = in.readLine.split(" ").map(_.toInt)
  val ab = Iterator.continually(in.readLine)
    .take(n)
    .map(_.split(" ").map(_.toLong))
    .toArray
    .sortBy(e => e(0))
  
  var ans = 0L
  var cnt = 0L
  for(i <- 0 until n if cnt < m) {
    if(cnt+ab(i)(1) < m) {
      ans += ab(i)(0) * ab(i)(1)
      cnt += ab(i)(1)
    }
    else {
      val rem = m - cnt
      ans += ab(i)(0) * rem
      cnt += rem
    }
  }
  // println(m + " " + cnt)
  println(ans)
}