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.Scanner;
import java.util.TreeMap;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		TreeMap <Integer, Integer> store = new TreeMap <Integer, Integer>();
		int Ai=0;
		
		for (int i = 0; i<N; i++){
			Ai = sc.nextInt();
 
			if (store.containsKey(Ai)) 
				store.put(Ai, store.get(Ai) + 1);
			 else 
				store.put(Ai, 1);
		}
		int a=0;
		int freq = 0;
		while (store.isEmpty()!=true && freq<2){
			a = store.lastKey();
			freq = store.get(store.lastKey());
			store.remove(a, freq);
		}
	
		
		int b=0;
		int freq2 = 0;
		while (store.isEmpty()!=true && freq2<2){
			b = store.lastKey();
			freq2 = store.get(store.lastKey());
			store.remove(b, freq2);
		}
	
		System.out.println(a*b);
		sc.close();
	}

}