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.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.Scanner;

public class Main{
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		long N = sc.nextInt();
		long M = sc.nextInt();
		
		List<Shop> shops = new ArrayList<Shop>();
		
		for(int i = 0; i < N; i++) {
			shops.add(new Shop(sc.nextLong(), sc.nextLong()));
		}
		
		Comparator<Shop> comp = new Comparator<Shop>() {
			@Override
			public int compare(Shop o1, Shop o2) {
				if(o1.money == o2.money) return 0;
				return o1.money < o2.money ? -1 : 1;
			}
		};
		
		Collections.sort(shops, comp);
		
		long res = 0;
		
		loop: for(Shop shop : shops) {
			for(int i = 0; i < shop.stock; i++) {
				if(M  > 0) {
					res+=shop.money;
					M-=1;
				}else{
					break loop;
				}
			}
		}
		
		System.out.println(res);
		
	}
}

class Shop{
	public long money;
	public long stock;
	
	public Shop(long money, long stock) {
		this.money = money;
		this.stock = stock;
	}
}