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.*;
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
  int N = sc.nextInt();
  long[] len = new long[1000000001];
  for(int i = 0;i < N;i++){
    int a = sc.nextInt();
    len[a]++;
  }
  long w = 0;
  long h = 0;
  int count = 0;
  out:
  for(int i = 1000000000;;i--){
    if(len[i] == 4 && count == 0){
      w = i;
      h = i;
      break;
    }
    else if(len[i] >= 2){
      if(count == 0){
        w = i;
        count++;
      }
      else if(count == 1){
        h = i;
        break out;
      }
    }
  }
  System.out.println(h*w);
}}