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)

//65 - A 90 - Z 97 - a 122 - z
#include <bits/stdc++.h>
using namespace std;

typedef long long LL;

unordered_map<int, int> mp;
int mx1, mx2;

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    int n; cin >> n;
    for(int i = 0; i < n; i++)
    {
        int a; cin >> a;
        mp[a] ++;
    }
    for(auto x: mp)
    {
        if(x.second == 2 || x.second == 3)
        {
            if(x.first > mx1)
            {
                mx2 = mx1;
                mx1 = x.first;
            }
            else if(x.first > mx2) mx2 = x.first;
        }
        if(x.second > 3)
        {
            if(x.first > mx1)
                mx1 = x.first, mx2 = x.first;
            else if(x.first > mx2) mx2 = x.first;
        }
    }
    LL res = (LL)mx1 * (LL)mx2;
    cout << res;
    return 0;
}