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 <bits/stdc++.h>

using namespace std;

int main(){
    int N,A,sz;
    vector<int> vec;
    cin >> N;
    unordered_map<int,int> mp;
    for(int i=0;i<N;i++){
        cin >> A;
        if(mp[A]==0){
            mp[A] =1;
        }else{
            vec.push_back(A);
            mp[A] = 0;
        }
    }

    sz = vec.size();

    if(sz<2){
        cout << "0" << endl;
    }else{
        sort(vec.begin(),vec.end());
        cout << vec[sz-1]*vec[sz-2] << endl;
    }
}