def issorted(l):
	return all(l[i] <= l[i+1] for i in range(len(l)-1))

def main():
	n, k = map(int, input().split())
	a = list(map(int, input().split()))
	b = [xx[0] for xx in sorted(enumerate(a), key=lambda x:x[1])]

	f = all((b[i] % k)==(i % k) for i in range(n))
	if not f:
		print(-1)
		return

	count = 0
	while not issorted(b):
		for i in range(n-k):
			if b[i] > b[i+k]:
				b[i], b[i+k] = b[i+k], b[i]
				count += 1
	print(count)

if __name__ == '__main__':
	main()