def is_sorted(A, N):
    for i in range(N - 1):
        if A[i] > A[i + 1]:
            return False
            break
    else:
        return True

def k_bubblesort(A, N, K, i):
    cnt = 0
    end = max(j for j in range(N) if j % K == i)

    for j in range(i, N, K):
        for l in range(end, j + K - 1, -K):
            if A[l - K] > A[l]:
                A[l - K], A[l] = A[l], A[l - K]
                cnt += 1

    return cnt

N, K = map(int, input().split())

A = [int(a) for a in input().split()]
tot_cnt = 0

for i in range(min(K, N)):
    tot_cnt += k_bubblesort(A, N, K, i)

# print(A)

if is_sorted(A, N):
    print(tot_cnt)
else:
    print(-1)