You are given a string S of length N consisting of lowercase characters 'a' and 'b'. In one operation, you can select a character and make it equal to one of its adjacent characters. For example, if S = "aab", in one operation you can convert it to any of the following:1. "aab": By changing the 1st character to the 2nd character.2. "aab": By changing the 2nd character to the 1st character.3. "abb": By changing the 2nd character to the 3rd character.4. "aaa": By changing the 3rd character to the 2nd character. Find the minimum number of operations to make all the characters in the string equal.