def sort_counter(lis):
    count = 0
    while True:
        innner_counter = 0
        for i in range(len(lis) - 1):
            if lis[i] > lis[i + 1]:
                lis[i], lis[i + 1] = lis[i + 1], lis[i]
                innner_counter += 1
        if innner_counter == 0:
            break
        else:
            count += innner_counter
    return count

N, K = map(int, input().split(' '))
g = [[] for i in range(K)]
for i, n in enumerate(map(int, input().split(' '))):
    g[i % K].append(n)
flag = True
for n in zip(*list(map(lambda l: sorted(l), g))):
    tn = n[0]
    for ttn in n[1:]:
        if tn <= ttn:
            tn = ttn
            continue
        flag = False
        break
    if flag == False:
        break

if flag:
    count = 0
    for l in g:
        count += sort_counter(l)
    print(count)
else:
    print(-1)