def kyori(s, t)
s.split('').zip(t.split('')).inject(0){|x,y| y[0] == y[1] ? x : x += (y[0] =~ /[aiueo]/ ? 2 : 1) }
end

kyori('kasa', 'sasa')
=> 1

kyori('kasa', 'kasu')
=> 2