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型オブジェクトの比較関数 ---*/
int compare_int(const void *a, const void *b)
{
    return *(int*)a - *(int*)b;
}

int main(void)
{
    int n;
    unsigned long long int a[100010];
    int i;
    scanf("%d", &n);
    for(i = 0; i < n; i++){
        scanf("%ld", &a[i]);
    }
    qsort(a, n, sizeof(int), compare_int);
    int x[2] = {};
    int temp = -1;
    int j = 0;
    for(i = n-1; i >= 0; i--){
        if(temp == a[i]){
            x[j] = a[i];
            temp = -1;
            j++;
            if(j == 2){
                break;
            }
        }else{
            temp = a[i];
        }
    
    }
    printf("%ld", x[0] * x[1]);
    
    return 0;
}