L, N = map(int, input().split())
X = list(map(int, input().split()))
pre0 = [0] * (L + 1)
pre1 = [0] * (L + 1)
pre1[X[0]] = 1
XX = []
X.append(L)
for i in range(N):
    XX.append(X[i + 1] - X[i])
for x in XX:
    dp0 = [0] * (L + 1)
    dp1 = [0] * (L + 1)
    for i in range(L + 1):
        if i - x >= 0:
            dp1[i] |= pre1[i - x]
            dp1[i] |= pre0[i - x]
        dp0[i] |= pre1[i]
        dp0[i] |= pre0[i]
    pre0, dp0 = dp0, pre0
    pre1, dp1 = dp1, pre1

ans = 10 ** 18
for i in range(L + 1):
    if pre0[i]:
        ans = min(ans, abs(2 * i - L))
        
print(ans)