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.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    HashMap<Integer, Integer> map = new HashMap<>();
    for (int i = 0; i < N; i++) {
      int tmp = sc.nextInt();
      if (map.containsKey(tmp)) {
        map.put(tmp, map.get(tmp) + 1);
      } else {
        map.put(tmp, 1);
      }
    }
    ArrayList<Integer> list = new ArrayList<>();
    ArrayList<Integer> list2 = new ArrayList<>();
    Set keys = map.keySet();
    for (Object key : keys) {
      if (map.get(key) >= 2) {
        list.add((Integer)key);
      }
      if (map.get(key) >= 4) {
        list2.add((Integer)key);
      }
    }
    Collections.sort(list);
    Collections.sort(list2);
    int size = list.size();
    int size2 = list2.size();
    if (size >= 2 && size2 >= 1) {
      System.out.println(Integer.max(list.get(size - 1) * list.get(size - 2), list2.get(size2 - 1) * list2.get(size2 - 1)));
    } else if (size >= 2 && size2 == 0) {
      System.out.println(list.get(size - 1) * list.get(size - 2));
    } else if (size == 0) {
      System.out.println(0);
    }
  }
}