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)

class ShopInformation(val unitPrice: Long, val limit: Long)

fun main(args: Array<String>) {
    val (n,m) = readLine()!!.split(" ").map { it.toInt() }
    val shops = Array(n) { readLine()!!.split(" ").map { it.toLong() }.let { ShopInformation(it.first(), it.last()) } }.sortedBy { it.unitPrice }
    var count = 0L
    var answer = 0L

    for (i in 0 until n) {
        answer += Math.min(shops[i].limit, m - count) * shops[i].unitPrice
        if (shops[i].limit >= m - count) break
        count += shops[i].limit
    }
    println(answer)
}