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 Java (OpenJDK 11.0.6) (judge verdict: Runtime Error)

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		
		HashMap<String,Long>drinkHashMap=new HashMap<>();
		
		Scanner sc=new Scanner(System.in);
		
		//栄養ドリンクが売られているお店の数
		long N=sc.nextLong();
		//集めたい栄養ドリンクの個数
		long M=sc.nextLong();
		long count=0;
		long sum=0;
	
		for(int i=0;i<N;i++) {
			//ドリンクの価格
			String A=sc.next();
			//買える本数
			long B=sc.nextLong();
			
			drinkHashMap.put(A,B);
		}
		
		
		List<String>listKey=new ArrayList<String>(drinkHashMap.keySet());
		Collections.sort(listKey);
	
		for(int i=0;count<M;i++) {
			long a=Long.parseLong(listKey.get(i));
			long b=drinkHashMap.get((String.valueOf(a)));
			for(int j=1;j<=b;j++){
				if(count<M) {
					count++;
					sum+=Long.parseLong(listKey.get(i));
				}else{
					break;
				}
			}
		}
		
		System.out.println(sum);
	}

}