def count_inv(A):
    n = len(A)
    res = 0
    for i in range(n):
        for j in range(i+1, n):
            res += A[i] > A[j]
    return res


N, K = map(int, input().split())
A = list(map(int, input().split()))
AA = [0] * N

ans = 0
for i in range(K):
    B = []
    for j in range(i, N, K):
        B.append(A[j])
    ans += count_inv(B)
    for j, b in enumerate(sorted(B)):
        AA[K*j+i] = b

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