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.Arrays;
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();
		Main main = new Main();
		Store store[] = new Store[N];
		for (int i=0;i<N;i++) {
			Store temp = main.new Store();
			temp.A = sc.nextLong();
			temp.B = sc.nextInt();
			store[i] = temp;
		}
		Arrays.sort(store);
		long sum =0;
		int count=0;
		for (int i=0;i<N;i++) {
			if (count +store[i].B>=M) {
				sum += (M-count)*store[i].A;
				break;
			} else {
				sum +=store[i].B*store[i].A;
				count +=store[i].B;
			}
		}
		System.out.println(sum);
	}

	public class Store implements Comparable<Store> {
		long A;
		int B;
		@Override
		public int compareTo(Store o) {
			if (this.A>o.A)
				return 1;
			if (this.A==o.A)
				return 0;
			return -1;
		}

	}
}