#include <iostream>
#include<fstream>
using namespace std;
ifstream f ("factorial.in");
ofstream g("factorial.out");
int nr_cif_0 (int n)
{
    int nr=0;
    while(n%10==0)
    {
        nr++;
        n=n/10;
    }
    return nr;
}
int factorial (int n)
{
    int i,P=1;
    if(n==0)
        return 1;
    else
        {for(i=1;i<=n;i++)
            P=P*i;
        return P;
        }
}
int main()
{
    int p,i=0;
    f>>p;
    do
    {
        i++;
    }while(nr_cif_0(factorial(i))<p);
    if(nr_cif_0(factorial(i))==p)
        g<<i;
    else
        g<<-1;
    cout << "Hello world!" << endl;
    return 0;
}