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 <algorithm>
#include <map>
using namespace std;

int main()
{
    int N;cin >>N;
    unsigned long long int vec[N];
    vector<unsigned long long int> as;
    for(int a = 0;a < N;a++){
       // cout <<a<<endl;
        cin >> vec[a];
    }
    cout << "finish!"<<endl;
    map<unsigned long long int,int> s;
    for(int a = 0;a < N;a++){
        if(s.find(vec[a]) != s.end()){
            s[vec[a]]++;
        }else{
            s[vec[a]] = 1;
        }
    }
    as.push_back(0);
    as.push_back(0);
    for(auto mp:s){
        for(int a = 0;a < mp.second / 2;a++){
            as.push_back(mp.first);
        }
    }
    std::sort(as.begin(),as.end());
    /*
    for(int a :as){
        cout<<a<<" ";
    }
    */
    unsigned long long int a = as.at(as.size()-1) * as.at(as.size()-2);
    cout << a<<endl;
    return 0;
}