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)

#include <iostream>
#include <vector>
#include <unordered_map>
#include <map>
#include <algorithm>
#include <set>
#include <unordered_set>
#include <deque>
#include <cmath>
#include <stack>
#include <queue>
#include <string>
#include <locale>
#include <iomanip>

#define nl '\n'

using namespace std;

long long n, a, x, f, s;
bool e;
 
int main(){

    cin >> n;

    set<long long> t;
    set<long long> s;

    for(long long i = 0; i < n; i++){
        cin >> a;
        x = t.size();
        t.insert(a);
        if(t.size() == x){
            e = true;
            s.insert(a);
        }
    }

    if(e){
        set<long long>:: iterator it = s.end();

        --it;
        cout << *it * *--it;
    } else{
        cout << 0;
    }
 
}

//g++ -std=c++14 comp.cpp -o b; ./b.exe