Possible error in README.md

I'm a little puzzled about the example in README.md - the `|ys|` contains four pairs `{ha,al,ll,lo}` yet in the calcs that follow this it's treated as if it has only size 3. Am I misunderstanding something, or should the calculation instead be like the below?

        s = (2*2)/(4+4) = 4/8 = 0.5