class FenwickTree:

    def __init__(self, N):
        self.N = N
        self.d = [0] * N
    
    def add(self, p, x):
        assert 0 <= p < self.N
        p += 1
        while p <= self.N:
            self.d[p - 1] += x
            p += p & (-p)
    
    def sum0(self, r):
        s = 0
        while r > 0:
            s += self.d[r - 1]
            r -= r & (-r)
        return s
    
    def get_sum(self, l, r):
        assert 0 <= l <= r <= self.N
        return self.sum0(r) - self.sum0(l)


N, M = map(int, input().split())
P = list(map(lambda x: x - 1, map(int, input().split())))
fw = FenwickTree(N)

invs = 0
for i in range(N):
    invs += fw.get_sum(P[i], N)
    fw.add(P[i], 1)
if invs == 0:
    print(0)
elif M % 2 == 0 and invs % 2 == 1:
    print(-1)
else:
    t = M * ((invs + M - 1) // M)
    if t % 2 == invs % 2:
        print(t)
    else:
        print(t + M)