n, k = map(int, input().split())
a = list(map(int, input().split()))
b = sorted(a)
c = 0
for i in range(1000):
  for j in range(n - k):
    if a[j] > a[j + k]:
      a[j], a[j + k] = a[j + k], a[j]
      c += 1
print(c if a == b else -1)