n, m = map(int, input().split())

d = [0] * m
for i in range(m):
    d[i] = int(input())

dist = 0
for i in range(n):
    p = d[i]
    q = d[i+n-1]
        
    if p * q > 0:
        dist = min(abs(p), abs(q))
    else:
        dist = abs(p) + abs(q) + min(abs(p), abs(q))
        
print(dist)