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.math.BigDecimal;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Scanner;
import java.util.stream.Collectors;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		BigDecimal sum = BigDecimal.ZERO;
		Map<Integer, Integer> map = new HashMap<>();
		for(int i =0; i < n; i++)
			map.put(sc.nextInt(), sc.nextInt());
		
		List<Entry<Integer, Integer>> list = map.entrySet().stream()
				.sorted(Comparator.comparingInt(entry -> entry.getKey()))
				.collect(Collectors.toList());
		
		for(Entry<Integer, Integer> entry : list) {
			if(m == 0)
				break;
			
			BigDecimal value = BigDecimal.valueOf(entry.getKey());
			BigDecimal num = BigDecimal.valueOf(entry.getValue());

			if(m > entry.getValue()) {
				m-= entry.getValue();
				sum = sum.add(value.multiply(num));
			}else if(m <= entry.getValue()) {
				num = BigDecimal.valueOf(m);
				BigDecimal b = value.multiply(num);
				sum = sum.add(b);
				
				m = 0;
			}
		}
		
		System.out.println(sum.toString());
	}
}