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

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

    def sum(self, l, r):
        assert 0 <= l <= r <= self._n
        return self._sum(r) - self._sum(l)

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

    def get_size(self, x):
        x = self.find(x)
        while r > 0:
            s += self.data[r - 1]
            r -= r & -r
        return s

    def get(self, k):
        k += 1
        x, r = 0, 1
        while r < self._n:
            r <<= 1
        len = r
        while len:
            if x + len - 1 < self._n:
                if self.data[x + len - 1] < k:
                    k -= self.data[x + len - 1]
                    x += len
            len >>= 1
        return x


N, M = map(int, input().split())
P = list(map(int, input().split()))
T = Fenwick_Tree(N + 1)
cnt = 0
for i in range(N):
    cnt += T.sum(P[i], N + 1)
    T.add(P[i], 1)
    
if cnt % 2:
    if M % 2 == 0:
        print(-1)
    else:
        ans = (cnt + M - 1)//M * M
        if ans % 2 == 0:
            ans += M
        print(ans)
else:
    ans = (cnt + M - 1)//M * M
    if ans % 2:
        ans += M
    print(ans)