Construct a regular expression that defines the language L containing all words beginning with exactly double a or exactly double b$.$ That is$,$ L contains all words that start with only $2$ a$$s or b$$s$.$ Words in L are at least of length $2$ and words such as aa$,$ bb$,$ bbababa and aabaabbabbbabb do belong to L; words such as a$,$ b$,$ babababa, bbbababa and aaaabbabbbabb do not belong to L$.$