# -*- coding: utf-8 -*-

N = input()
K = input()
Alist = [i+1 for i in range(0, N)]
Blist = [i+1 for i in range(0, N)]
Klist = [0 for i in range(0, K)]
for i in range(0, K):
    Klist[i] = map(int, raw_input().split())[0] - 1
for i in Klist:
        Alist[i], Alist[i+1] = Alist[i+1], Alist[i]
flag = 0
cnt = 1
while Alist != Blist:
    cnt += 1
    for i in Klist:
        Alist[i], Alist[i+1] = Alist[i+1], Alist[i]
print cnt