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: Runtime Error)

import java.util.List;
import java.util.Arrays;
import java.util.Map;
import java.util.HashMap;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        int m = sc.nextInt();

        Map<Long, Long> drink = new HashMap<>();
        for (int i = 0; i < n; i++) {
            drink.put(sc.nextLong(), sc.nextLong());
        }

        Object[] drink_money = drink.keySet().toArray();
        Arrays.sort(drink_money);
        long bottle = 0;
        long money = 0;


        for (int i = 0; i < n; i++) {
            long d_money = Integer.valueOf(String.valueOf(drink_money[i]));
            long d_bottle = Integer.valueOf(String.valueOf(drink.get(d_money)));
            if (bottle + d_bottle <= m) {
                System.out.println(d_money + " " + d_bottle);
                bottle += d_bottle;
                money += d_money * d_bottle;
            } else {
                System.out.println(d_money + " " + (m - bottle));
                money += d_money * (m - bottle);
                break;
            }
        }
        System.out.println(money);

    }
}