N, M = map(int, input().split())
A = list(map(int, input().split()))
ans = N - M
for i in range(M - 1) :
    n = A[i + 1] - A[i] - 1
    mn = 1
    while n > 1 and n & 1 :
        ans -= mn
        n >>= 1
        mn <<= 1
print(ans)