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

    def sum(self, i):
        s = 0
        while i > 0:
            s += self.tree[i]
            i -= i & -i
        return s

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

N, M = map(int, input().split())
P = list(map(int, input().split()))
bit = Bit(N + 1)
cnt = 0
for i, p in enumerate(P):
    bit.add(p, 1)
    cnt += (i + 1) - bit.sum(p)
tmp = - (- cnt // M)
if cnt % 2 == 1 and M % 2 == 0:
    print(-1)
elif (tmp * M - cnt) % 2 == 0:
    print(tmp * M)
else:
    print((tmp + 1) * M)