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

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

    def sum(self, l, r):
        '''範囲[l, r)(lからr-1まで)の総和を求める'''
        return self._sum(r) - self._sum(l)

    def _sum(self, r):
        '''範囲[0, r)(0からr-1まで)の総和を求める'''
        s = 0
        while r > 0:
            s += self.data[r - 1]
            r -= r & -r
        return s

n, m = map(int, input().split())
A = list(map(int, input().split()))

cnt = 0
FT = Fenwick_Tree(n)
for i in range(n):
    a = A[i] - 1
    cnt += FT.sum(a, n)
    FT.add(a, 1)
if cnt % 2 == 1 and m % 2 == 0:
    print(-1)
elif cnt % m == 0:
    print(cnt)
else:
    r = m - cnt % m
    if r % 2 == 0:
        print(cnt + r)
    else:
        print(cnt + r + m)