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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<bits/stdc++.h>
using namespace std;
int main()
{
    long long n,i,j,k,l,a,b,d,s,c;
    cin>>n;
    long long arr[n+10];
    for(i=0;i<n;i++)
    {
        cin>>arr[i];
    }
    s=-100000;
    sort(arr+0,arr+n);

    for(i=n-1;i-3>=0;i--)
    {
        a=arr[i];
        b=arr[i-1];
        c=arr[i-2];
        d=arr[i-3];
        k=0;
        l=0;
        if(a==b&&b==c&&c==d)
        {
            k=a*b;
        }
        if(a==b&&c==d&&a!=c)
        {
            l=a*c;
        }
        if(k<l)
        {
            k=l;
        }
        if(k>s)
        {
            s=k;
        }
    }
    cout<<s<<endl;
    return 0;

}