rectangle.cpp

// NOTE: it is recommended to use this even if you don't understand the following code.

#include <fstream>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
    // uncomment the two following lines if you want to read/write from files
    // ifstream cin("input.txt");
    // ofstream cout("output.txt");

    int N;
    cin >> N;
    
    vector<long long> S(N);
    for (int i = 0; i < N; ++i)
        cin >> S[i];
    
    long long A = 0;
    
    
    // INSERT YOUR CODE HERE
    sort (S.begin(), S.end());
    reverse (S.begin(), S.end());

    vector<long long> a(0);
    for (int i=0; i<N; i++) {
        if (i != N-1) if (S[i] == S[i+1]) {
            a.push_back (S[i++]);
            if (a.size() == 2) break;
        }
    }
    
    if (a.size() == 2) A = a[0] * a[1];
    else A = 0;
    
    cout << A << endl;

    return 0;
}