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 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<stdio.h>
#include<stdlib.h>
int asc(const void *a, const void *b)
{
    return *(int*)b - *(int*)a;
}
int main()
{
    int n;
    scanf("%d", &n);
    int i;
    int a[100005];
    for (i = 0; i < n; i++)
        scanf("%d", &a[i]);
    qsort(a, n, sizeof(int), asc);
    double x, y;
    x = y = 0;
    for (i = 0; i < n - 1; i++)
        if (a[i] == a[i + 1])
        {
            x = a[i];
            break;
        }
    i += 2;
    for(;i<n-1;i++)
        if (a[i] == a[i + 1])
        {
            y = a[i];
            break;
        }
    printf("%.0lf\n", x*y);
    return 0;
}