class BIT:
    def __init__(self, n):
        self.size = n
        self.tree = [0] * (n + 1)

    def sum(self, i):
        # 配列0番目からiまでの総和を出力
        # i: 1-indexed
        s = 0
        while i > 0:
            s += self.tree[i]
            i -= i & -i
        return s

    def add(self, i, x):
        # i番目にxを可算
        while i <= self.size:
            self.tree[i] += x
            i += i & -i

    def update(self, i, x):
        # i番目を加算処理でxに更新する
        self.add(i, -self.sum(i) + self.sum(i - 1))
        self.add(i, x)

    def debug(self):
        # 現在の配列状況を出力する
        res = []
        for i in range(self.size):
            res.append(self.sum(i + 1) - self.sum(i))
        return res


def inversion_number(A):
    b = BIT(N)
    invert = 0
    for i in range(N):
        invert += b.sum(N) - b.sum(A[i])
        b.add(A[i], 1)
    return invert


N, M = map(int, input().split())
P = list(map(int, input().split()))
inv = inversion_number(P)
moyori = M * -(-inv // M)
# print(inv, moyori)
if (moyori - inv) % 2 == 0:
    print(moyori)
else:
    print(-1)