l, n = map(int, input().split())
X = list(map(int, input().split()))

X = [0] + X + [l]
D = []
for i in range(n + 1):
    D.append(X[i + 1] - X[i])
a = D[0]
b = D[-1]
D = D[1:-1]
l = sum(D)
DP = [[False for _ in range(l + 1)] for _ in range(n)]
DP[0][0] = True
for i in range(n - 1):
    for j in range(l + 1):
        DP[i + 1][j] |= DP[i][j]
        if j + D[i] <= l:
            DP[i + 1][j + D[i]] |= DP[i][j]

ans = 10**18
for i in range(l + 1):
    if DP[n - 1][i]:
        ans = min(ans, abs(a + i - b - (l - i)))
print(ans)