L, N = map(int, input().split())
X = [0] + list(map(int, input().split())) + [L]
A = []
for i in range(N + 1):
    A.append(X[i + 1] - X[i])
N = len(A)
M = 2 * L + 10
dp = [[[0] * 2 for _ in range(M)] for _ in range(N + 1)]
dp[0][L][0] = 1
for i, a in enumerate(A):
    for j in range(M):
        for k in range(2):
            if not dp[i][j][k]:
                continue
            if not k:
                dp[i + 1][j + a][k] = 1
                if i:
                    dp[i + 1][j - a][1 - k] = 1
            else:
                dp[i + 1][j - a][k] = 1
                if i:
                    dp[i + 1][j + a][1 - k] = 1
ans = 10 ** 18
for j in range(M):
    if dp[N][j][1]:
        ans = min(ans, abs(j - L))
print(ans)
# for i in range(N + 1):
#     print(dp[i])