var
	fi, fo : text;
  n, nf5, p, p5 : integer;
begin
	assign(fi,'fact.in'); reset(fi);
  assign(fo,'fact.out'); rewrite(fo);
  read(fi,p);
  n := 5*p;
  p5 := 25;
  while p5 <= n do
  	begin
  		n := n-5*(n div p5-1);
  		p5 := p5*5;
    end;
  write(fo,n);
  close(fi); close(fo);
end.