N,K=map(int,raw_input().split())
a = map(int,raw_input().split())
c = {v:i for i,v in enumerate(sorted(a))}
for i in xrange(N):
	a[i] = c[a[i]]
ans = 0
for i in xrange(N):
	q,r = divmod(a.index(i)-i,K)
	if r != 0:
		ans = -1
		break
	ans += q
	for j in xrange(i+q*K,i,-K):
		a[j],a[j-K] = a[j-K],a[j]
print ans