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.*;

public class Main {
	static class Shop implements Comparable<Shop> {
		long A;
		int B;

		Shop(long A, int B) {
			this.A = A;
			this.B = B;
		}

		@Override
		public int compareTo(Shop o) {
			if (this.A < o.A) {
				return -1;
			} else if (this.A > o.A) {
				return 1;
			} else
				return 0;
		}
	}

	int N, M;
	long[] A;
	int[] B;
	Queue<Shop> shops;

	Main() {
		Scanner in = new Scanner(System.in);
		N = in.nextInt();
		M = in.nextInt();
		A = new long[N];
		B = new int[N];
		for (int i = 0; i < N; ++i) {
			A[i] = in.nextLong();
			B[i] = in.nextInt();
		}
		in.close();
		this.shops = new PriorityQueue<>(N);
		for (int i = 0; i < N; ++i) {
			this.shops.add(new Shop(A[i], B[i]));
		}
	}

	long calc() {
		long result = 0;
		int remain = M;
		while (remain > 0) {
			Shop shop = this.shops.poll();
			result += Math.min(remain, shop.B) * shop.A;
			remain -= shop.B;
		}
		return result;
	}

	public static void main(String[] args) {
		Main ins = new Main();
		System.out.println(ins.calc());
	}

}