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: Accepted)

#include <stdlib.h>
#include <stdio.h>
int compare(const void *a,const void *b){
    return *(long long*)b - *(long long*)a ;
}
int main()
{
    long long n,h=0,i,w=0,flag=0,a[100001];
    scanf("%lld",&n);
    for(i=0;i<n;i++){
        scanf("%lld",&a[i]);
    }
    qsort(a,n,sizeof(long long),compare);
    for(i=0;i<n-1;i++){
        //printf("a[%lld]=%lld\n",i,a[i]);
        if(a[i]==a[i+1]&&flag==0){
            h=a[i];
            i++;
            flag=1;
        }else if(a[i]==a[i+1]){
            w=a[i];
            break;
        }
        
    }
    printf("%lld",h*w);
    return 0;
}