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: Accepted)

#include <iostream>
#include <vector>
#include <algorithm>

#define REP(i, n) for(int i = 0;i < n;i++)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define ALL(a) (a).begin(), (a).end()

using namespace std;

int main()
{
    int N; cin >> N;

    vector<int> A(N);

    REP(i, N)
    {
        cin >> A[i];
    }

    sort(ALL(A), greater<>());

    vector<int> arr;

    int prev_key = -1;
    int prev_n = 0;
    REP(i, N)
    {
        if( prev_key == A[i] )
        {
            prev_n++;
        }
        else
        {
            if( prev_n >= 4 )
            {
                arr.push_back(prev_key);
                arr.push_back(prev_key);
            }
            else if( prev_n >= 2 )
            {
                arr.push_back(prev_key);
            }

            prev_n = 1;
            prev_key = A[i];
        }
    }
    if( prev_n >= 4 )
    {
        arr.push_back(prev_key);
        arr.push_back(prev_key);
    }
    else if( prev_n >= 2 )
    {
        arr.push_back(prev_key);
    }

    if( arr.size() < 2 )
    {
        cout << 0 << endl;
    }
    else
    {
        cout << ((unsigned long) arr[0]) * arr[1] << endl;
    }

    return 0;
}