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(' '))
w = list(map(int, input().split(' ')))
g = [[] for i in range(K)]
for i, n in enumerate(w):
    g[i % K].append(n)
flag = True

if w != sorted(w):
    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)
        if count != 0:
            print(count)
        else:
            print(-1)
    else:
        print(-1)
else:
    print(0)