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.Scanner;
import java.util.TreeSet;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc=new Scanner(System.in);
		// Problem C
		int n = sc.nextInt();
		TreeSet<Integer> s = new TreeSet<Integer>();
		int first = 0;
		int second = 0;
		for(int i=0;i<n;i++) {
			int tmp = sc.nextInt();
			if (s.contains(tmp)) {
				if(tmp >= first) {
					second = first;
					first = tmp;
					s.remove(tmp);
				} else if (tmp >= second) {
					second = tmp;
					s.remove(tmp);
				}
			} else {
				s.add(tmp);
			}
		}
		System.out.println((long)first * (long)second);
	}

}