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.*;
import java.util.stream.Collectors;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int a[] = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = Integer.parseInt(sc.next());
        }
        sc.close();
        Map<Integer, Integer> h = new TreeMap<>(Comparator.reverseOrder());
        for (int i = 0; i < n; i++) {
            if (h.containsKey(a[i])) {
                h.put(a[i], h.get(a[i]) + 1);
            } else {
                h.put(a[i], 1);
            }
        }
        List<Integer> max4 = h.entrySet().stream().filter(x -> x.getValue() >= 4).map(x -> x.getKey())
                .collect(Collectors.toList());
        List<Integer> max2 = h.entrySet().stream().filter(x -> x.getValue() >= 2).map(x -> x.getKey())
                .collect(Collectors.toList());
        long ans = 0;
        if (max4.size() > 0) {
            ans = (long) max4.get(0) * max4.get(0);
        }
        if (max2.size() > 1) {
            ans = Math.max(ans, (long) max2.get(0) * max2.get(1));
        }
        System.out.println(ans);
    }
}