Liars

$N$ people at an algorithm camp gather at the front of the classroom and sit in a circle. They are numbered $0$ through $N-1$ counterclockwise.

Every participant is either honest or a liar. An honest person always tells the truth, and a liar always tells a lie.

The host asked each person whether the person sitting to their right is a liar. An honest person answers truthfully and a liar answers falsely. Both an honest person and a liar may refuse to answer.

Given the answers, write a program that prints the smallest possible number of liars if some assignment of honest people and liars produces exactly those answers, and $-1$ if no assignment does.

The first line contains the number of people $N$. ($2 \le N \le 50$)

The second line contains the answers as a string of length $N$. The $i$-th character from the left, counting from $0$, is the answer of person $i$.

An answer of L means the person said the one to their right is a liar, H means the person said that one is honest, and ? means the person refused to answer.

If some assignment of honest people and liars produces the given answers, print the smallest possible number of liars. Otherwise print $-1$.

The numbering runs counterclockwise, so the person sitting to the right of person $i$ is person $(i+1) \bmod N$.