Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

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 in = new Scanner(System.in);
		int n = in.nextInt();
		int[] a = new int[n];
		for(int i=0;i<n;i++) {
			a[i] = in.nextInt();
		}
		Arrays.sort(a);
		long longest = 0;
		long longer = 0;
		int i = n-1;
		for(;i>=1;i--) {
			if(a[i]==a[i-1]) {
				longest = a[i];
				i -= 2;
				break;
			}
		}
		for(;i>=1;i--) {
			if(a[i]==a[i-1]) {
				longer = a[i];
				i--;
				break;
			}
		}
		System.out.println(longer*longest);
		in.close();
	}

}