Devu and his Class

Devu is a class teacher of a class of n students. One day, in the morning prayer of the school, all the students of his class were standing in a line. You are given information of their arrangement by a string s. The string s consists of only letters 'B' and 'G', where 'B' represents a boy and 'G' represents a girl. Devu wants inter-gender interaction among his class should to be maximum. So he d