class FenwickTree:
    def __init__(self, n: int):
        self.data = [0] * (n+10)
        self.n = len(self.data) - 1

    def add(self, p: int, x: int):
        assert 0 <= p < self.n
        p += 1
        while p < len(self.data):
            self.data[p] += x
            p += p & -p

    def sum(self, p: int) -> int:
        """区間 [0, p] の和"""
        assert 0 <= p < self.n
        p += 1
        s = 0
        while p > 0:
            s += self.data[p]
            p -= p & -p
        return s

    def range_sum(self, l: int, r: int) -> int:
        """区間 [l, r] の和"""
        assert 0 <= l <= r < self.n
        s = self.sum(r)
        if l > 0:
            s -= self.sum(l-1)
        return s

    def suffix_sum(self, l: int) -> int:
        """区間 l... の和"""
        return self.range_sum(l, self.n-1)


def inversion_count(a: list[int]) -> int:
    """転倒数を求める"""
    ft = FenwickTree(max(a) + 1)
    res = 0
    for i, x in enumerate(a, 1):
        ft.add(x, 1)
        res += i - ft.sum(x)
    return res


def solve():
    slis = sorted(A)

    res = 0
    for i in range(K):
        xs = []
        ys = []
        for j in range(i, N, K):
            xs.append(A[j])
            ys.append(slis[j])

        if sorted(xs) != ys:
            return -1
        if len(xs) > 0:
            res += inversion_count(xs)

    return res


N, K = map(int, input().split())
A = list(map(int, input().split()))

ans = solve()
print(ans)