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: Wrong Answer)

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 = Integer.parseInt(sc.next());

		int[] A = new int[N+1];
		for(int i =1; i < N+1; i++) {
			A[i] = Integer.parseInt(sc.next());
		}
		sc.close();

		Arrays.sort(A);

//		int max = 0;
//		for(int i = 1; i <A.length; i++) {
//			max = Math.max(max, A[i]);
//		}
//
//		int[] numbers = new int[max + 1];
//		for(int i = 1; i < A.length; i++) {
//			numbers[A[i]]++;
//		}

		int ans = 1;
		int count = 0;
		int prev = A[N];
		for(int i = A.length-2; i > 0; i--) {
			if(A[i] == prev) {
				ans *= prev;
				count ++;
				prev = 0;
			}else {
				prev = A[i];
			}
			if(count == 2) {
				System.out.println(ans);
				break;
			}
		}

		if(count != 2) System.out.println(0);

//		int l1 = 0, l2 = 0;
//		for(int i = numbers.length-1; i > 0; i-- ) {
//			if(numbers[i] >= 4 && l1 == 0) {
//				l1 = i;
//				break;
//			}else if(numbers[i] >= 2 && l1 == 0) {
//				l1 = i;
//			}else if(numbers[i] >= 2 && l1 != 0) {
//				l2 = i;
//				break;
//			}
//		}

//		if(l1 != 0 && l2 != 0) {
//			System.out.println(l1 * l2);
//		}else if(l1 != 0 && l2 == 0) {
//			System.out.println(l1 * l1);
//		}else {
//			System.out.println(0);
//		}

	}

}