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.Arrays;
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<Long,Integer> Map=new HashMap<Long,Integer>();

		for(int i=0;i<N;i++) {
			Map.put(sc.nextLong(),sc.nextInt() );
		}

		Object[] mapkey=Map.keySet().toArray();
		Arrays.sort(mapkey);

		int now_Drink=0;
		int Drink_count=0;
		long ans=0;


		for (Long nKey : Map.keySet())
        {

			now_Drink=Map.get(nKey);

			if(M < Drink_count+now_Drink) {
				now_Drink=(M-Drink_count);
			}

			ans+=(long)(now_Drink*nKey);
			Drink_count+=now_Drink;

			if(Drink_count==M) break;

        }

		System.out.println(ans);

		sc.close();
	}

}