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: Runtime Error)

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[] A = new int[N];
		
		for(int i = 0; i < N; i++) {
			A[i] = sc.nextInt();
		}
		
		Arrays.sort(A);
		
		int[] B = new int[A[N - 1] + 10];
		
		for(int i = 0; i < N; i++) {
			B[A[i]]++;
		}
		
		long S = 0;
		
		for(int i = 0; i < B.length; i++) {
			if(B[i] >= 2) {
				for(int j = i + 1; j < B.length; j++) {
					if(B[j] >= 2) {
						S = Math.max(S, i * j);
					}
				}
				if(B[i] >= 4) {
					S = Math.max(S, i * i);
				}
			}
		}
		System.out.println(S);
	}
}