class Fenwick_Tree:
    def __init__(self, n):
        self.n = n
        self.data = [0] * n

    def add(self, p, x):
        p += 1
        while p <= self.n:
            self.data[p - 1] += x
            p += p & -p

    def sum(self, l, r):
        '''範囲[l, r)(lからr-1まで)の総和を求める'''
        return self._sum(r) - self._sum(l)

    def _sum(self, r):
        '''範囲[0, r)(0からr-1まで)の総和を求める'''
        s = 0
        while r > 0:
            s += self.data[r - 1]
            r -= r & -r
        return s

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

from bisect import bisect_left
def compression(lst):
    sort_lst = sorted(set(lst))
    compression_lst = [None for _ in range(len(lst))]
    ele2ind_dict = dict()
    for i, ele in enumerate(lst):
        compression_lst[i] = bisect_left(sort_lst, ele)
        ele2ind_dict[ele] = compression_lst[i]
    return sort_lst, compression_lst, ele2ind_dict
_, _, ele2ind_dict = compression(A)
for i in range(n):
    A[i] = ele2ind_dict[A[i]]

ans = 0
if n > k:
    for i in range(k):
        FT = Fenwick_Tree(n)
        for j in range(i, n, k):
            a = A[j]
            ans += FT.sum(a, n)
            FT.add(a, 1)
        AA = A[i::k]
        AA.sort()
        idx = 0
        for j in range(i, n, k):
            A[j] = AA[idx]
            idx += 1
for i in range(n - 1):
    if A[i] > A[i + 1]:
        ans = -1
        break
print(ans)