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>

int main(){
    int n;
    scanf("%d", &n);
    unsigned a[n-1];
    unsigned c=0, b=0, k;

    for(int i=0; i<n; i++){
        scanf("%u", &a[i]);
        for(int j=i; a[j]>a[j-1] || j>0; j--){
            k = a[j-1];
            a[j-1] = a[j];
            a[j] = k;
        }
    }
        for(int i=0; i<n; i++){
        if(a[i]==a[i+1]){
            if(c==0){
                c = a[i];
            }
            if(c != 0 && a[i] != c){
                b = a[i];
                break;
            }
        }
    }
    if(b==0){
        printf("%llu", (unsigned long long)c^2);
    }else{
        printf("%llu", (unsigned long long)b*c);
    }
    return 0;
}