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<stdio.h>
#include<stdlib.h>
#define N 100005
long long fun(long long [],long long );
int cmp(const void *,const void *);
int cmp(const void *a,const void *b)
{
    return *(int *)a-*(int *)b;
}
long long fun(long long a[],long long n)
{
    long long i;
    long long cnt[3]={0},t=0;
    qsort(a,n,sizeof(a[i]),cmp);
    for(i=n-1;i>=1;i--)
    {
        if(a[i]==a[i-1])
        {
            cnt[t++]=a[i];
            i--;
            if(t==2)
            {
                break;
            }
        }
    }
    return cnt[0]*cnt[1];
}
int main()
{
    long long n,i;
    long long a[N];
    scanf("%lld",&n);
    for(i=0;i<n;i++)
    {
        scanf("%lld",&a[i]);
    }
    printf("%lld\n",fun(a,n));
    return 0;
}