def ok(l):
    for i in range(len(l) - 1):
        if l[i] > l[i + 1]:
            return False
    return True


def func(a: list, k):
    if len(a) == 0:
        return 0
    ans = 0
    index = a.index(min(a))
    if index % k != 0:
        print(-1)
        exit()

    while index - k >= 0:
        ans += 1
        a[index], a[index - k] = a[index - k], a[index]
        index -= k

    ans += func(a[1:], k)
    return ans


n, k = map(int, input().split())
a = list(map(int, input().split()))

print(func(a, k))