l, n = map(int, input().split())
X = list(map(int, input().split()))
A = []
for i in range(n - 1):
    A.append(X[i + 1] - X[i])
tf = [False] * (l + 1)
tf[0] = True
for a in A:
    for i in range(l, a - 1, -1):
        tf[i] |= tf[i - a]

aa = X[0]
bb = l - X[-1]
ans = l
for i in range(l):
    if tf[i]:
        a_ = aa + i
        b_ = l - a_
        ans = min(ans, abs(a_ - b_))
print(ans)