#include <iostream>
#include <fstream>
using namespace std;
int nrCifre(int nr)
{
    int numarCifre=0;
    while(nr!=0 && nr%10==0)
    {
        nr/=10;
        numarCifre++;
    }
    return numarCifre;
}
int main()
{
    int p;
    ifstream f("factorial.in");
    ofstream g("factorial.out");
    f>>p;
    int i=1;
    int nr;
    int j=1;
    while(nr!=p)
    {
        j=j*i;
        i++;
        nr=nrCifre(j);
    }
    g<<i-1;
    return 0;
}