import itertools
N, K = map(int, input().split())
block = list(map(int, input().split()))
count = 0

def sort(b):
    global count
    k = len(b) - 1
    for i in range(k):
        for j in range(k, i, -1):
            if b[j - 1] > b[j]:
                count += 1
                temp = b[j]
                b[j] = b[j - 1]
                b[j - 1] = temp
    return b

def is_sorted(result):
    if len(result) == 1:
        return True
    m = 0
    for e in result:
        if e < m:
            return False
        m = e
    return True

if N <= K:
    if is_sorted(block):
        print(0)
    else:
        print(-1)
else:
    div = []
    for start in range(K):
        itr = itertools.islice(block, start, None, K)
        div.append(list(itr))
        
    sort_div = []
    for d in div:
        sort_div.append(sort(d))
    
    done_swap = [list(z) for z in itertools.zip_longest(*sort_div)]
    done_swap = [n for n in sum(done_swap,[]) if n != None]
    if is_sorted(done_swap):
        print(count)
    else:
        print(-1)