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: Wrong Answer)

import java.util.*;
public class Main {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int M = sc.nextInt();
		
		int [] A = new int [N];
		int [] B = new int [N];


		for ( int i = 0; i < N; i++) {
				A[i] = sc.nextInt();
				B[i] = sc.nextInt();
			}
		int sum = 0;
		int rest = M;
		int min;

		Outer:
		for (int j = 0; j < A.length; j++) {
			min = A[0];
			for ( int i = 0; i < A.length; i++) {
				min = Math.min(A[i], min);
			}
		
			for ( int i = 0; i < A.length; i++) {
				if (min == A[i]) {
					if (rest >= B[i]) {
						rest -= B[i];
						sum += A[i] * B[i];	
						A[i] = 1000000000;
					}
					else {
						sum += A[i] * rest;
						break Outer;
					}
				
				}	
			}
			
		}
		
		System.out.println(sum);
	}
}