const
  pow5: array[2..25] of qword = (25, 125, 625, 3125, 15625, 78125, 390625,
                                 1953125, 9765625, 48828125, 244140625,
                                 1220703125, 6103515625, 30517578125,
                                 152587890625, 762939453125, 3814697265625,
                                 19073486328125, 95367431640625,
                                 476837158203125, 2384185791015625,
                                 11920928955078125, 59604644775390625,
                                 298023223876953125);

var
  n, count: qword;
  i: byte;

begin
  readln(n);
  count := n div 5;
  for i := 2 to 25 do
    count := count + n div pow5[i] * (i - 1);
  writeln(count)
end.