n,k,*a=gets(p).split.map(&:to_i)
ans=0
loop{
	swapped=false
	0.upto(n-k-1){|i|
		if a[i]>a[i+k]
			swapped=true
			ans+=1
			a[i],a[i+k]=a[i+k],a[i]
		end
	}
	break unless swapped
}
puts a.each_cons(2).all?{|i,j|i<j} ? ans : -1