n = int(input())

if n < 2:
  print(1)
elif n % 2 == 1:
  print(0)
  exit()
else:
  if n > 9:
    a = n // 10
  else:
    print(0)
    exit()
  
  def calc(a,n):
    num = 50
    while n > num:
      a += n//num
      num *= 5
    return a
  
  print(calc(a,n))