... I know it's 42-14 = 28, but I don't know why...?

I'm guessing that 42 is the number of balanced strings for n=5, and 14 is the 4th catalan number, but I'm not sure WHY you have to subtract the 4th catalan number in order to get how many strings end in RR.

I'm a beginner to discrete mathematics and would really appreciate some help... cheers. =)