N,K,*a=$<.read.split.map &:to_i
r=0
N.times{
	(N-K).times{|i|
		a[i]>a[i+K]&&(a[i],a[i+K]=a[i+K],a[i];r+=1)
	}
}
p (0..N-2).all?{|i|a[i]<a[i+1]}?r:-1