from copy import deepcopy
import typing


class FenwickTree:
    '''Reference: https://en.wikipedia.org/wiki/Fenwick_tree'''

    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

N, K = map(int, input().split())
A = list(map(int, input().split()))
B = sorted(list(set(A)))
D = dict(zip(B, range(len(B))))

for i in range(N):
    A[i] = D[A[i]]

B = [[] for _ in range(K)]
for i in range(N):
    B[i % K].append(A[i])
C = deepcopy(B)

for b in B:
    b.sort()
now = 0
for i in range(N):
    b = B[i % K][i // K]
    if now > b:
        print(-1)
        exit()
    now = b

ans = 0
for cs in C:
    if cs:
        NC = max(cs)
        ft = FenwickTree(NC + 1)
        cnt = 0
        for c in cs:
            cnt += ft.sum(c, NC + 1)
            ft.add(c, 1)
        ans += cnt
print(ans)