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 java.util.Scanner

import scala.annotation.tailrec


object Main {

  case class Store(price: Long, purchaseLimit: Int)

  def read() = {
    val sc = new Scanner(System.in)
    val nStores, nNecessaryDrinks = sc.nextInt()
    val stores: IndexedSeq[Store] = for (i <- 0 until nStores) yield {
      val price, purchaseLimit = sc.nextInt()
      Store(price.toLong, purchaseLimit)
    }
    (nStores, nNecessaryDrinks, stores)
  }

  @tailrec
  def purchase(totalPrice: Long, nNecessaryDrinks: Int, stores: List[Store]): Long = {
    stores match {
      case store :: _ if nNecessaryDrinks <= store.purchaseLimit =>
        totalPrice + store.price * nNecessaryDrinks
      case store :: restStores =>
        purchase(totalPrice + store.price * store.purchaseLimit,
          nNecessaryDrinks - store.purchaseLimit,
          restStores)
    }
  }

  def solve(nStores: Int, nNecessaryDrinks: Int, stores: IndexedSeq[Store]): Long = {
    // 値段でソートする
    val sortedStores = stores.sortBy(store => store.price)

    purchase(0, nNecessaryDrinks, sortedStores.toList)
  }

  def main(args: Array[String]): Unit = {
    val (nStores, nNecessaryDrinks, stores) = read()
    println(solve(nStores, nNecessaryDrinks, stores))
  }
}