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: Wrong Answer)

import scala.collection.immutable.TreeMap

object Main {
  def main(args:Array[String]) = {
    val sc = new java.util.Scanner(System.in)
    val n = sc.nextInt
    val m = sc.nextInt
    val map : TreeMap[Int,Int] = (1 to n).foldLeft[TreeMap[Int,Int]](TreeMap.empty)((map, _) => {
      val x = sc.nextInt
      val y = sc.nextInt
      map.updated(x, map.getOrElse(x,0) + y)
    })

    val end : (BigDecimal,Int) = map.foldLeft[(BigDecimal,Int)]((0, m)){case ((sum, toBuy),(x,y)) => {
      if(toBuy == 0) (sum,0)
      else{
        val buying = Math.min(toBuy, y)
        (sum + x*buying, toBuy - buying)
      }
    }}
    println(end._1)
  }

}