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)

//omajinai
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
//omajinai

int main(){

    // omajinai
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    // omajinai

    ll n;
    cin >> n;
    vector<ll> a(n);
    for(int i = 0;i < n;i ++){
        cin >> a.at(i);
    }

    sort(a.begin(),a.end(),greater<ll>());

    ll x=0,y=0,z;

    for(ll i = 0;i < n-1;i ++){
        if(a.at(i) == a.at(i+1)){
            x = a.at(i);
            z = i + 2;
            break;
        }
    }

    for(ll i = z;i < n-1;i ++){
        if(a.at(i) == a.at(i+1)){
            y = a.at(i);
            break;
        }
    }

    cout << x * y << endl;
}