#include<iostream>
#include<fstream>
using namespace std;
int factorial(long n)
{
    long i,p=1;
    for(i=1;i<=n;i++)
    {
        p=p*i;
    }
    return p;
}
int zero(long n)
{
    int nr=0;
    while(n%10==0)
    {
        nr++;
        n=n/10;
    }
    return nr;
}
int main()
{
    long P,t=0,z=2,ok=1;
    ifstream fin("factorial.in");
    ofstream fout("factorial.out");
    fin>>P;
    while(t<=z)
    {
        t++;
        z++;
        if(zero(factorial(t))==P)
        {
            fout<<t;
            break;
        }
        if(z>=100)
        {
            ok=0;
            break;
        }
    }
    if(ok==0)
    {
        fout<<-1;
    }
    fin.close();
    fout.close();
    return 0;
}