DFA (Deterministic Finite Automaton)

Question

what would be regular expression to say : A deterministic automaton that is: $\Sigma = \{a, b\}$ that accepts strings of length at least $2$ consisting of either: all $a$’s or all $b$’s (but not containing both symbols). I am looking for answer in following form such as $a^*b^+c||c$ . This is just an indication. I have an idea if, $(aa^+\mid bb^+)$ is right expression...