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

def solve(N, K, As):
    Bs = sorted(As)
    slicedAs = slice_lst(As, K)
    slicedBs = slice_lst(Bs, K)
    cnt = 0
    for sAs, sBs in zip(slicedAs, slicedBs):
        if set(sAs) != set(sBs):
            return -1
        cnt += count_swaps(sAs)
    return cnt

def slice_lst(As, K):
    sliced = []
    for k in range(K):
        sliced.append(As[k::K])
    return sliced

def count_swaps(As):
    n = len(As)
    count = 0
    for i in range(n-1):
        for j in range(i+1, n):
            if As[i] > As[j]:
                count += 1
    return count

print(solve(N, K, As))