N, m = map(int, input().split())
P, M = [0], []
for i in range(m):
    D = int(input())
    if D >= 0:
        P.append(D)
    else:
        M.append(D)
inf = 10 ** 18
M.append(0)
M.reverse()
while len(P) < N + 1:
    P.append(inf)
while len(M) < N + 1:
    M.append(-inf)
ans = inf
for i in range(N + 1):
    ans = min(ans, 2 * P[i] - M[N - i])
    ans = min(ans, -2 * M[i] + P[N - i])
print(ans)