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 $2a^{\text{'}}s$ or $b\text{'}$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 $L.$