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: Time Limit Exceeded)

import java.util.ArrayList;
import java.util.Collections;
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.nextLong();
		long M = sc.nextLong();

		List<Drink> drinkList = new ArrayList<>();
		sc.nextLine();
        for (int i = 0; i < N; i++) {
        	Drink drink = new Drink(sc.nextInt(), sc.nextInt());
        	drinkList.add(drink);
        	sc.nextLine();
        }

        long count = 0;
        long ansPrice = 0;
        List<Long> priceList = new ArrayList<>();
    	for(Drink drink : drinkList){
        	priceList.add(drink.A);
        }
    	Collections.sort(priceList);
        while(true){
        	Drink drink = drinkList.stream().filter(drnk -> drnk.A == priceList.get(0)).findFirst().get();
        	if(M > count + drink.B) {
        		count += drink.B;
        		ansPrice += drink.A * drink.B;
        	}else{
        		ansPrice += drink.A * (M- count);
        		break;
        	}

        	drinkList.remove(drinkList.indexOf(drink));
        	priceList.remove(0);
        }

        System.out.println(ansPrice);

	}


}

class Drink {

	Drink(long a, long b){
		this.A = a;
		this.B = b;
	}

	long A;
	long B;
}