#include<fstream.h>
#include<math.h>
main()
  {ifstream f("fact.in");
  ofstream g("fact.out");
  unsigned long long q,c,i,inf,p[1000],sup;
  f>>q;
  inf=1; sup=9999999999999;
  p[1]=1;
  do
    {p[i]=p[i-1]*5+1;
     i++;}
  while(q>p[i]);

  do {c=(inf+sup)/2;
      if (p[c]<q) inf=c+1;
         else sup=c-1;}
  while(inf!=sup);
  g<<c;}