N, M = map(int, input().split())
A = list(map(int, input().split()))
A = [0]+A+[N+1]
M += 2

ans = 0
for i in range(M-1):
    diff = A[i+1]-A[i]-1
    ans += diff
    if diff%2 == 1:
        cnt = 0
        while diff >= 2 and diff%2 == 1:
            ans -= 2**cnt
            diff //= 2
            cnt += 1

print(ans)