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.Arrays;
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);

	    int N=Integer.parseInt(scanner.next());
	    int M=Integer.parseInt(scanner.next());
	    long[] A=new long[N];
	    long[] B=new long[N];
	    Map<Long, Long> map = new HashMap<Long, Long>();


	    for(int i=0; i<N;i++) {
	    	A[i]=Long.parseLong(scanner.next());
	    	B[i]=Long.parseLong(scanner.next());
	    	if (map.containsKey(A[i])) {
	    		map.put(A[i], map.get(A[i] + B[i]));
	    	} else {
	    		map.put(A[i], B[i]);
	    	}
	    }
	    
	    Arrays.parallelSort(A);
	    
	    long h=0;
	    long c=0;
	    for (int i=0; i<N; i++) {
	    	if (i!=0 && A[i] == A[i-1]) continue;
	    	
	    	long hm = map.get(A[i]);
	    	for (int j = 0; j < hm; j++) {
	    		h++;
	    		c+=A[i];
	    		if (h==M) {
	    			System.out.println(c);
	    			return;
	    		}
	    	}
	    }

	}
}