#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    ifstream in ("fact.in");
    int p, n;
    in >> p;
    if(p %2 == 0)
    {
        n = (p/2)*10;
    }
    else
        if(p != 0)
        n = (p/2)*10 + 5;
        else
            n = -1;
    ofstream out("fact.out");
    out << n;
    in.close();
    out.close();
    return 0;
}
//Se citeste P. sa se afle N minimum astfel incat N! are P zerouri la final