A Game of Strings

Alice and Bob have invented a new game. Alice gives Bob a string consisting of only two types of characters i.e. $'a'$ and $'b'$. Bob can do a special operation on this string called a $flip$. If Bob flips from index $i$ to $j$ (inclusive of i and j) , all the $'a'$ in this range become $'b'$ and all the $'b'$ become $'a'$. Help Bob find the maximum number of $'a'$, in this string after