def ordinal_suffix(n):
  """Return the ordinal suffix for a positive integer
  >>> ordinal_suffix(0)
  ''
  >>> ordinal_suffix(1)
  'st'
  >>> ordinal_suffix(2)
  'nd'
  >>> ordinal_suffix(3)
  'rd'
  >>> ordinal_suffix(4)
  'th'
  >>> ordinal_suffix(11)
  'th'
  >>> ordinal_suffix(12)
  'th'
  >>> ordinal_suffix(13)
  'th'
  >>> ordinal_suffix(21)
  'st'
  >>> ordinal_suffix(22)
  'nd'
  >>> ordinal_suffix(23)
  'rd'
  >>> ordinal_suffix(101)
  'st'
  >>> ordinal_suffix(111)
  'th'
  >>> ordinal_suffix(112)
  'th'
  >>> ordinal_suffix(113)
  'th'
  >>> ordinal_suffix(121)
  'st'
  >>> ordinal_suffix(1111)
  'th'
  >>> ordinal_suffix(1322)
  'nd'
  >>> ordinal_suffix('horse')
  ''
  """
  try:
      n = int(n)
  except Exception:
      return ''
  if n < 1:
    return ''
  elif n >= 100:
    return ordinal_suffix(n%100)
  elif 11 <= n <= 13:
    return 'th'
  elif n%10 in (1,2,3):
    return ('st','nd','rd')[n%10-1]
  else:
    return 'th'