#include <fstream>

using namespace std;

long long c,p,n,k,aux;

ifstream f("fact.in");
ofstream g("fact.out");

int main()
{
    f>>p;
    if (p==0) n++;
    c=0;
    n=0;
    while (c<p)
    {
        n=n+5;
        k=1;
        aux=n;
        while (aux)
        {
            aux=aux/5;
            if (aux%5==0) k++;
            else break;
        }
        c=c+k;
    }
    if (c>p) n=-1;
    g<<n;
    f.close();
    g.close();
    return 0;
}