Banana String

Gru has a string $S$ of length $N$, consisting of only characters $a$ and $b$ for banana and $P$ points to spend. Now Gru wants to replace and/or re-arrange characters of this given string to get the lexicographically smallest string possible. For this, he can perform the following two operations any number of times. 1) Swap any two characters in the string. This operation costs $1$ $point$.