Chef and String

There are $N$ students standing in a row and numbered $1$ through $N$ from left to right. You are given a string $S$ with length $N$, where for each valid $i$, the $i$-th character of $S$ is 'x' if the $i$-th student is a girl or 'y' if this student is a boy. Students standing next to each other in the row are friends. The students are asked to form pairs for a dance competition. Each pair must