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: Wrong Answer)

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

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        long N = scanner.nextLong(), M = scanner.nextLong();
        Map<Long, Long> drinks = new HashMap<>();
        for(int i = 0; i < N; i++){
            Long key = scanner.nextLong();
            Long value = scanner.nextLong();
            drinks.computeIfPresent(key,(k, v) -> v + value);
            drinks.computeIfAbsent(key,(k) -> value);
        }
        drinks.entrySet().stream().sorted((m1, m2) -> {
            return m1.getValue().compareTo(m2.getValue());
        });
        long ans = 0;

        for(Long k : drinks.keySet()){
            long buy = Long.min(drinks.get(k), M);
            ans += (buy * k);
            M -= buy;
            if(M == 0)
                break;
        }
        System.out.println(ans);
    }
}