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.*;
import java.lang.*;

class Pair implements Comparable<Pair>{
  long a;
  int cnt;
  public Pair(long i, int j){
    this.a = i;
    this.cnt = j;
  }
  public int compareTo(Pair p){
    if(this.cnt>=2 && p.cnt >=2 && this.a<p.a) return -1;
    if(this.cnt<2 && p.cnt>=2) return -1;
    return 1;
  }
}

class Main{
  public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      int N = Integer.parseInt(sc.next());
      HashMap<Long,Pair> map = new HashMap<>();
      ArrayList<Pair> l = new ArrayList<>();
      for(int i = 0; i<N; i++){
        long c = Long.parseLong(sc.next());
        if(!map.containsKey(c)){
           Pair p = new Pair(c,1);
           map.put(c, p);
           l.add(p);
        }else{
          Pair p = map.get(c);
          p.cnt++;
        }
      }
      Collections.sort(l);
      int size = l.size();
      Pair p1 = l.get(size-1);
      Pair p2 = l.get(size-2);
      long res = 0;
      if(p1.cnt > 2 && p2.cnt>2) res = p1.a*p2.a;
      System.out.println(res);
  }
}