Convertor $(100$ Marks$)$

You are given a string $S$ containing two characters $\text{'}a\text{'}$ and $\text{'}b\text{'}.$ In one operation, you can two non$-$equal indexes in the string and swap the characters. You have to tell the minimu operations needed to make the string non$-$decreasing.

We define a string as non$-$decreasing if $S[i]S[i+1],$ holds for every based