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 C (Clang 3.8.0) (judge verdict: Time Limit Exceeded)

#include <stdio.h>

    void bsort(long a[], int n)
    {
        int i, j;
        for (i = 0; i < n - 1; i++) {
            for (j = n - 1; j > i; j--) {
                if (a[j - 1] > a[j]) {
                    long temp = a[j];
                    a[j] = a[j - 1];
                    a[j - 1] = temp;
                }
            }
        }
    }

    int main(void)
    {
        int n;
        scanf("%d", &n);
        long a[n];
        int i;
        for (i = 0; i < n; i++)
            scanf("%ld", &a[i]);
        
        bsort(a, n);

        int fir = 0;
        int sec = 0;
        for (i = n - 1; ; i--) {
            if (a[i] == a[i - 1]) {
                fir = a[i];
                break;
            }
        }
        i -= 2;

        while (i + 1) {
            if (a[i] == a[i - 1]) {
                sec = a[i];
                break;
            }
            i--;
        }
        
        printf("%d\n", fir * sec);

        return 0;
    }