string_2, cat,dog

[Dixon]

count_cat = 0
  count_dog = 0
  for i in range(len(str)-2):
    if str[i:i+3] == 'dog':
      count_dog += 1
    if str[i:i+3] == 'cat':
      count_cat += 1
   
  return count_cat == count_dog

Can someone explain the purpose of the -2 in the for loop?