n,k = map(int,raw_input().split())
a = map(int,raw_input().split())
b = [sorted(a).index(i) for i in a]
cnt = 0
for i in range(n):
	if b[i]%k != i%k:
		cnt = -1
if cnt == 0:
	for i in range(n):
		while True:
			j = b.index(i)
			if i != j:
				cnt += 1
				c = cmp(i,j)
				b[j], b[j+k*c] = b[j+k*c], b[j]
			else:
				break
print cnt