N,K=map(int,raw_input().split())
a = map(int,raw_input().split())
for o,(p,v) in enumerate(sorted(enumerate(a),key=lambda x:x[1])):
	a[p] = o
i = ans = 0
while i < N:
	if a[i] == i:
		i+=1
		continue
	if i-a[i] % K != 0:
		ans = -1
		break
	j=i
	while a[j] != j:
		a[j],a[j+K] = a[j+K],a[j]
		j += K
		ans += 1
print ans