n,k=map(int,raw_input().split())
a=map(int,raw_input().split())
c=0
for i in range(k):
	for j in range(i,n,k):
		for l in range(j+k,n,k):
			if a[j]>a[l]:
				a[j],a[l]=a[l],a[j]
				c+=1
print[-1,c][a==sorted(a)]