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.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Map;
import java.util.Scanner;
import java.util.HashMap;
import java.util.Comparator;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        A solver = new A();
        solver.solve(1, in, out);
        out.close();
    }

    static class A {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            // 入力
            int n = Integer.parseInt(in.next());
            Integer[] a = new Integer[n];
            Map<Integer, Integer> kinds = new HashMap<>();
            for (int i = 0; i < n; i++) {
                a[i] = Integer.parseInt(in.next());
                if (kinds.containsKey(a[i] - 1)) {
                    kinds.put(a[i] - 1, kinds.get(a[i] - 1) + 1);
                } else {
                    kinds.put(a[i] - 1, 1);
                }
            }
            long ans = 0;
            Arrays.sort(a, Comparator.reverseOrder());
            for (int i = 0; i < n; i++) {
                if (kinds.get(a[i] - 1) >= 4) {
                    ans = a[i] * a[i];
                    out.println(ans);
                    return;
                } else if (kinds.get(a[i] - 1) >= 2) {
                    int tmp = a[i];
                    for (int j = i + 1; j <= n - 1; j++) {
                        if (a[j - 1] != a[j] && kinds.get(a[j] - 1) >= 2) {
                            ans = a[j] * tmp;
                            out.println(ans);
                            return;
                        }
                    }
                }
            }

            out.println(ans);

        }

    }
}