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)

fun main (args : Array<String>) {
	val (n, m) = readLine()!!.split(" ").map(String::toInt)
	val N : Int = n.toInt()
	var M : Long = m.toLong()
	var A : Long
	var B : Long
	val AList : MutableList<Long> = mutableListOf()
	val BList : MutableList<Long> = mutableListOf()
	var NAList : MutableList<Long> = mutableListOf()
	var result : Long = 0
	var minA : Long
	for (i in 0..N - 1) {
		var (a, b) = readLine()!!.split(" ").map(String::toInt)
		A = a.toLong()
		B = b.toLong()
		AList.add(A)
		NAList.add(A)
		BList.add(B)
	}
	NAList.sort()
	for (i in 0..N - 1) {
		minA = NAList[i]
		for (j in 0..N - 1) {
			if (minA == AList[j]) {
				if (BList[j] < M) {
					result += minA * BList[j]
					M -= BList[j] 
				} else if (BList[j] == M) {
					result += minA * BList[j]
					return println("${result}")
				} else {
					result += minA * M
					return println("${result}")
				}
			}
		}
	}
}