program infoarena_factorial;
uses math;
 var p,n,y,p5:int64;
   f:text;
begin

  assign(f,'fact.in');reset(f);readln(f,p);close(f);
  assign(f,'fact.out');rewrite(f);
  {
  write('p=');
  readln(p);    }
  if p=0 then begin n:=1;write(f,n);close(f);end
  else begin
  n:=0;
  while p>0 do
begin
  n:=n+5;
  y:=n;
  p5:=0;
  while y mod 5=0 do
  begin
    p5:=p5+1;
    y:=y div 5;
    end;
  p:=p-p5;
end;
end;   write(f,n);close(f); { write(n);readln; }

end.