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: Accepted)

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Scanner;

class AB {
	private Long fA;
	private int fB;

	public AB(Long a, int b) {
		fA = a;
		fB = b;
	}

	public Long getA() {
		return fA;
	}

	public int getB() {
		return fB;
	}

	public void setB(int b) {
		fB = b;
	}
}

class compAB implements Comparator<AB> {
	public int compare(AB ab1, AB ab2) {
		if (ab1.getA() < ab2.getA()) {
			return -1;
		} else if (ab1.getA() > ab2.getA()) {
			return 1;
		} else {
			return 0;
		}
	}
}

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int M = sc.nextInt();
		ArrayList<AB> ABList = new ArrayList<AB>(N);
		for (int i = 0; i < N; i++) {
			Long a = sc.nextLong();
			int b = sc.nextInt();
			ABList.add(new AB(a, b));
		}

		Collections.sort(ABList, new compAB());

		long c = 0;
		int id = 0;
		for (int j = 0; j < M; j++) {
			c += ABList.get(id).getA();
			ABList.get(id).setB(ABList.get(id).getB() - 1);
			if (ABList.get(id).getB() <= 0) {
				id++;
			}
		}

		System.out.println(c);

		sc.close();
	}
}