n, k = map(int, input().split())
A = list(map(int, input().split()))
ans = 0
for s in range(k):
    g = s
    while g < n:
        g += k
    for i in range(s, g, k):
        for j in range(g - k, i, -k):
            if A[j] < A[j - k]:
                A[j - k], A[j] = A[j], A[j - k]
                ans += 1

if A != sorted(A):
    print(-1)
else:
    print(ans)