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):
        """return sum(a[l:r-1])"""
        assert 0 <= l and l <= r and 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


n, m = map(int, input().split())
p = list(map(int, input().split()))
ft = fenwick_tree(n)
l = 0
for i in range(n):
    l += ft.sum(p[i], n)
    ft.add(p[i] - 1, 1)

if l % 2 == 1 and m % 2 == 0:
    print(-1)
else:
    ans = 0 - -l // m * m
    if ans % 2 != l % 2:
        ans += m
    print(ans)