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 Kotlin (1.0.0) (judge verdict: Wrong Answer)

import java.math.BigInteger
import java.util.Scanner

fun main(args: Array<String>) {
    val sc = Scanner(System.`in`)
    val shopCount = sc.nextInt()
    val drinkCount = sc.nextBigInteger()
    val shops = parseList(sc, shopCount)
    val result = buy(drinkCount, shops)
    println(result)
}

private fun parseList(scanner: Scanner, length: Int): List<Shop> {
    return (0 until length).map {
        val price = scanner.nextBigInteger()
        val size = scanner.nextBigInteger()
        Shop(price, size)
    }.sortedBy { it.price }
}

private fun buy(targetDrinkCount: BigInteger, shops: List<Shop>, cost: BigInteger = BigInteger.ZERO, drinkCount: BigInteger = BigInteger.ZERO): BigInteger {
    val minShop = shops.first()
    val shouldBuyCount = targetDrinkCount - drinkCount
    if (shouldBuyCount > minShop.size) {
        // 全て買う
        val tempCost = minShop.price * minShop.size
        return buy(targetDrinkCount, shops.drop(1), cost + tempCost, drinkCount + minShop.size)
    }
    // 買う個数が決まってる
    val tempCost = minShop.price * shouldBuyCount
    println("tempCost: $tempCost")
    return cost + tempCost
}

data class Shop(val price: BigInteger, val size: BigInteger)