import typing
class FenwickTree:

    def __init__(self, n: int = 0) -> None:
        self._n = n
        self.data = [0] * n

    def add(self, p: int, x: typing.Any) -> None:
        assert 0 <= p < self._n

        p += 1
        while p <= self._n:
            self.data[p - 1] += x
            p += p & -p

    def sum(self, left: int, right: int) -> typing.Any:
        assert 0 <= left <= right <= self._n

        return self._sum(right) - self._sum(left)

    def _sum(self, r: int) -> typing.Any:
        s = 0
        while r > 0:
            s += self.data[r - 1]
            r -= r & -r

        return s

# リストを受けて転倒数を出す関数
def count_inversion(li:list):
    # リストと要素数
    res = 0
    limit = max(li)
    ft = FenwickTree(limit + 1) # 一番右のしきり・・limit + 1
    for num in li:
        # 自分より右側にいる出席者数をカウントする（区間和）
        res += ft.sum(num + 1, limit + 1)
        # 自分も席に座る
        ft.add(num, 1)

    return res

def main():
    N, K = map(int, input().split())
    A = list(map(int, input().split()))
    data = [list() for _ in range(K)]
    for i, a in enumerate(A):
        data[i%K].append(a)

    B = [0]*N
    ans = 0
    for i in range(K):
        # data[i] の転倒数
        if data[i]:
            ans += count_inversion(data[i])
            data[i].sort()
            pos = 0
            for j in range(i, N, K):
                B[j] = data[i][pos]
                pos += 1
    A.sort()
    if A == B:
        print(ans)
    else:
        print(-1)
if __name__ == "__main__":
    main()