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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;

public class Main {

	public static Drink [] drinks;

	public static class Drink implements Comparable<Drink>{
		long cost;
		long drinks;
		double cperd;

		public Drink(){

		}

		public Drink(long cost_, long drinks_){
			cost = cost_;
			drinks = drinks_;
		}

		@Override
		public int compareTo(Drink o) {
			// TODO Auto-generated method stub
			return (int)(this.cost - o.cost);
		}

	}



	public static void main(String[] args) {
		// TODO Auto-generated method stub

		Scanner sc = new Scanner(System.in);
		long N = sc.nextInt();
		long M = sc.nextInt();
		long total = 0;
		PriorityQueue <Drink> q = new PriorityQueue<Drink>();
		for(long i = 0 ;i<N; i++) {
			q.add(new Drink(sc.nextInt(),sc.nextInt()));
		}

		while(M > 0) {
			Drink cur = q.poll();
			if(M>cur.drinks) {
				total += cur.drinks*cur.cost;
				M -= cur.drinks;
			}
			else{
				total += M*cur.cost;
				M = 0;
			}

		}
		System.out.println(total);

	}

}