Rikken have various numbers of limbs and are usually found in pairs. Rikken a will only pair with rikken b when the sum of their limbs has the same number of digits as a. For a given rikken (a) and a potential pair (b), return the sum of their limbs if a will pair with b, otherwise just return a without b. (Note: one way to compute the number of digits of a non-negative int n is to convert it to a string with String.valueOf(n) and then check the length of the string.)