from math import gcd

n, m = map(int, input().split())
p = list(map(int, input().split()))
c = 0
for i in range(n):
    if p[i] - 1 != i:
        c += 1
c = max(0, c - 1)
if gcd(2, m) != 1:
    print(-1)
else:
    n = (-c * pow(2, -1, m)) % m
    print((c + 2 * n))