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)

#define _USE_MATH_DEFINES // for C++  
#include "bits/stdc++.h"
#define LL unsigned long long
#define NUM 1000000007

using namespace std;

int main()
{
    LL n, ans1 = 0, ans2 = 0;
    cin >> n;
    map<LL, LL> m;
    for (size_t i = 0; i < n; i++)
    {
        LL tmp;
        cin >> tmp;
        auto iter = m.find(tmp);
        if (iter == m.end())
        {
            m.insert(make_pair(tmp, 1));
        }
        else
        {
            iter->second++;
        }
    }
    for (auto itrr = m.rbegin(); itrr != m.rend(); itrr++)
    {
        if (itrr->second >= 2 && ans1 == 0)
        {
            ans1 = itrr->first;
            continue;
        }

        if (ans1 != 0)
        {
            if (itrr->second >= 2)
            {
                ans2 = itrr->first;
                cout << ans1*ans2 << endl;
                return 0;
            }
        }
    }
    if (ans1 == 0 || ans2 == 0)
    {
        cout << 0 << endl;
    }

}