L, N = map(int, input().split())
X = list(map(int, input().split())) + [L]
Q = [{X[0]}, set()]

now = X[0]
for x in X[1:]:
    dif = x - now
    Q2 = [set(), set()]
    for j in range(2):
        for q in Q[j]:
            Q2[0].add(q + dif)
            Q2[1].add(q - dif)
    now = x
    Q = Q2
ans = 10 ** 10
for q in Q[1]:
    ans = min(ans, abs(q))
print(ans)