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 = sc.nextInt();
    HashMap<Long, Integer> map = new HashMap<Long, Integer>();
    for(int i = 0; i < n; i++) {
      long a = sc.nextLong();
      if(map.containsKey(a)) {
        map.put(a, map.get(a) + 1);
      } else {
        map.put(a, 1);
      }
    }
    ArrayList<Long> list = new ArrayList<Long>();
    long ans = 0;
    for(long key : map.keySet()) {
      int t = map.get(key);
      while(t > 1) {
        list.add(key);
        t -= 2;
      }
    }
    Collections.sort(list);
    if(list.size() > 1) {
      ans = (list.get(list.size() - 1)) * (list.get(list.size() - 2));
    }
    System.out.println(ans);
  }
}