#!/usr/bin/env python3
from sys import exit
s = int(input())
n = int(input())
if n < 1 or n > 100000: exit(1)
l = sorted(map(int, input().split()))
if len(l) != n: exit(2)
for i, e in enumerate(l):
    if i//2 != e - 1: exit(3)
exit(42)