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: Accepted)

fun main(args : Array<String>) {
    val (n, m) = readLine()!!.split(" ").map{it.toInt()}
    val shops = mutableListOf<Pair<Int, Int>>()

    repeat(n) {
        val (a, b) = readLine()!!.split(" ").map{it.toInt()}
        shops.add(Pair(a, b))
    }

    shops.sortBy { it.first }
    var cnt = 0
    var sum = 0L

    shops.forEach {
        cnt += it.second
        sum += it.first.toLong() * it.second.toLong()

        if (cnt > m) {
            val diff = cnt - m
            cnt -= diff
            sum -= diff.toLong() * it.first.toLong()
        }

        if (cnt == m) {
            println(sum)
            return
        }
    }
}