program infoarena_factorial;
uses math;
 var p,n,y:int64;
   gata:boolean;
   f:text;
begin
  assign(f,'fact.in');reset(f);readln(f,p);close(f);
  assign(f,'fact.out');rewrite(f);
  if p=0 then begin n:=1;write(f,n);close(f);end
  else begin
  n:=5;
  y:=round(int(log10(n)/log10(5)));
  gata:=(10*p=2*n+5*y*(y-1));
  while not gata do
begin
  n:=n+5;
  y:=round(int(log10(n)/log10(5)));
  gata:=(10*p=2*n+5*y*(y-1));
end;   write(f,n);close(f);
  end;
end.