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.*;

public class Main{
  public static void main(String[]args){
    Scanner sc = new Scanner(System.in);
    int N = Integer.parseInt(sc.nextLine());
    List<Long> list = new ArrayList<>();
    String[] S = sc.nextLine().split(" ");
    for(int i = 0; i < N; i++){
      long n = Long.parseLong(S[i]);
      list.add(n);
    }
    
    Collections.sort(list, Collections.reverseOrder());
    
    long len1 = 0;
    long len2 = 0;
    long tem = list.get(0);
    for(int i = 1; i < N; i++){
      if(len1 > 0 && len2 > 0){
        break;
      }
      
      if(tem == list.get(i)){
        if(len1 == 0){
          len1 = tem;
          i++;
          tem = list.get(i);
        }else{
          len2 = tem;
        }
      }else{
        tem = list.get(i);
      }
    }
    
    long ans = len1 * len2;
    System.out.println(ans);
  }
}