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();
		int[][] ab = new int[n][2];

		for (int i = 0; i < n; i++) {
			ab[i][0] = sc.nextInt();
			ab[i][1] = sc.nextInt();
		}
		//a(金額）の少ない順でソート
		Arrays.sort(ab, (a, b) -> Integer.compare(a[0], b[0]));

		long price = 0;
		long amount = 0;
		for (int i = 0; i < n; i++) {
			if (amount + ab[i][1] <= m) {
				price += (long)ab[i][0] * ab[i][1];
				amount += ab[i][1];
			} else {
				price += ab[i][0] * (m - amount);
				break;
			}
			//			System.out.println(price);
		}

		System.out.println(price);

		//		for (int i = 0; i < n; i++) {
		//			System.out.print(ab[i][0] + ", ");
		//			System.out.println(ab[i][1]);
		//		}

		sc.close();
	}
}