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: Runtime Error)

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
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();

		class DoubleInteger implements Comparable<DoubleInteger>{

			long a;
			long b;

			public int compareTo(DoubleInteger ab) {return ab.a<this.a ? 1 : ab.a==0 ? 0 : -1;}

			public DoubleInteger(long a, long b) {
				this.a=a;
				this.b=b;
			}

		}

		List<DoubleInteger> AB = new LinkedList<>();

		for(int i=0;i<N;i++) {

			long a =sc.nextInt();
			long b =sc.nextInt();
			AB.add(new DoubleInteger(a,b));

		}

		Collections.sort(AB);

		long result = 0;
      	int counter = 0;
		for(int i = 0;M>0;i++) {
			result += AB.get(i).a * AB.get(i).b;
			M -= AB.get(i).b;
          	counter++;
		}
		result -= (long)Math.abs(M) * AB.get(counter).a;
		System.out.println(result);

	}

}