Awkwardness Minimization

You are teaching a class of $N$ students. Today, during the morning prayer, all the students are standing in a line. You are given a string $s$ with length $N$; for each valid $i$, the $i$-th character of this string is 'b' if the $i$-th student in the line is a boy or 'g' if this student is a girl. The *awkwardness* of this configuration of students is defined as the sum of pairwise distances be