Minimum distance between two words in a string

gistfile1.txt

def distance(a,b)
  words = "I am a good girl".split(/\W+/)

  a_found = false
  b_found = false
  prev = nil

  words.each_with_index do |word, index|

    if word == a
      prev ||= index
      a_found = true
    end

    if word == b
      prev ||= index
      b_found = true
    end

    if a_found && b_found
      return index - prev
    end
  end
  -1

end