MasterChef

Chef has recently started conducting a masterclass where he teaches his disciples world-class recipes. There are $N$ students in his class, standing in a line represented by a string $S$ of length $N$. Each character in $S$ is either `B` (representing a boy) or `G` (representing a girl). Chef has an annoyance factor $K$. He gets annoyed and refuses to teach if and only if there are at least $K$