def check(a,b,N):
	for i in range(N):
		if a[i] not in b[i % K]:
			return False
	return True
N,K = map(int,input().split())
a = [int(i) for i in input().split()]
b = [set([]) for i in range(K)]
c = sorted([i for i in a])
for i in range(N):
	b[i % K].add(c[i])
if not check(a,b,N):
	print(-1)
else:
	cnt = 0
	while True:
		flag = True
		for i in range(N - 1):
			if a[i] > a[i + 1]:
				flag = False
		if flag:
			break
		for i in range(N):
			j = i + K
			while j < N:
				if a[j - K] > a[j]:
					a[j],a[j - K] = a[j - K],a[j]
					cnt+=1
				j += K
	print (cnt)