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 Java (OpenJDK 11.0.6) (judge verdict: Time Limit Exceeded)

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);

        int n = console.nextInt();
        int m = console.nextInt();
        console.nextLine();

        List<Long> priceList = new ArrayList<Long>(n);
        List<Long> numberList = new ArrayList<Long>(n);

        for(int i = 0; i < n; i++){
            priceList.add(console.nextLong());
            numberList.add(console.nextLong());
            console.nextLine();
        }

        long money = 0;
        while(m > 0){
            long minPrice = Collections.min(priceList);
            int minIndex = priceList.indexOf(Collections.min(priceList));
            long minNumber = numberList.get(minIndex);

            for(int i = 0; i < minNumber && m > 0; i++){
                money += minPrice;
                m--;
            }
            priceList.remove(minIndex);
            numberList.remove(minIndex);
        }
        System.out.println(money);
    }
}