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.*;
public class Main{
    public static void main(String[]args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        long[] a = new long[n];
        for(int i=0; i<n; i++){
            a[i] = sc.nextLong();
        }
        Arrays.sort(a);
        long side1 = 0;
        long side2 = 0;
        
        for(int i=n-1; i>0; i--){ //　i=0の時終了しないとa[i-1]を検証してしまう
            if(a[i] == a[i-1]){
                if(side1 == 0){
                    side1 = a[i];
                    i--;
                }else{
                    side2 = a[i];
                    break;
                }
            }
        }
        System.out.println(side1 * side2);
    }
}