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: Time Limit Exceeded)

#include <bits/stdc++.h>

using namespace std;

long long n,s,k,mx,fs,sc,i;
map <int,int> t;

int main() 
{
    cin>>n;
    for (i=1; i<=n; i++)
        cin>>k,t[k]++,mx=max(mx,k);
    for (i=mx; i>=1; i--)
        if (t[i]>=2) 
        {
            if (fs!=0) sc=i; else fs=i,i++,t[i-1]-=2;
            if (sc!=0) break;
        }
    cout<<fs*sc;
}