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 sc = new Scanner(System.in);
		
		int N = sc.nextInt();
		int M = sc.nextInt();
		
		Map<Integer, Integer> map = new HashMap<Integer, Integer>();
		
		for(int i = 0; i < N; i++) {
			map.put(sc.nextInt(), sc.nextInt());
		}
		
		map.entrySet().stream().sorted(java.util.Map.Entry.comparingByValue());

		long count = 0;
		long honsuu = 0;
		
		test: for(int i = 0; i < N; i++) {
			for(Map.Entry<Integer, Integer> entry : map.entrySet()) {
				for(int j = 0; j < entry.getValue(); j++) {
					if(honsuu < M) {
						honsuu++; 
						count+=entry.getKey();
					}else{
						break test;
					}
				}
			}
		}
		System.out.println(count);
	}
}