text: import sys
  
  
  n = int(sys.stdin.readline().strip())
  
  num = 1
  for i in range(1, n+1):
      num *= i
  
  cnt = 0
  for i in str(num)[::-1]:
      if i != '0':
          break
      cnt += 1
  
  sys.stdout.write(str(cnt))
  