var f,g:text;
  var p,n,mar:longint;
begin
   assign( f, 'fact.in');
   reset (f);
   read(f,p);
   close (f);
   n:=0;
   if p<>0 then
     while p<>1 do
     begin
       inc(n);
       p:= p div n;
       mar:=p mod n;
       if mar <>0 then begin
                        p:=1;
                        n:=-1;
                       end;
     end;
   assign(g,'fact.out');
   rewrite (g);
   write (g,n);
   close (g);
end.