n,k = [ int(v) for v in input().split() ]

number_list = [ int(v) for v in input().split() ]
number_list_sorted = sorted(number_list)

difference_list = [ abs(number_list_sorted.index(number_list[i])-i) for i in range(n) ]


difference_list = [ i % k == 0 for i in difference_list ]


if all(difference_list) == False:
    print(-1)
else:
    c = 0
    for i in range(n):
        target = number_list_sorted.index(number_list[i])
        temp = number_list[target]
        for j in range(target,i,-k):
            number_list[j] = number_list[j-k]
            c += 1
        number_list[i] = temp
    print(c)