import sys
sys.setrecursionlimit(5*10**5)
input = sys.stdin.readline
from collections import defaultdict, deque, Counter
from heapq import heappop, heappush
from bisect import bisect_left, bisect_right
from math import gcd

l,n = map(int,input().split())
x =[0] + list(map(int,input().split()))
x.append(l)
INF = 10**15
dp = [0]*((l*2+50)*2)
c = l+10
dp[c*2+1] = 1
for i in range(1,n+2):
    new = [0]*((l*2+50)*2)
    xi = x[i] - x[i-1]
    for j in range(l*2+50):
        for k in range(2):
            if k == 0:
                if j + xi < l*2+50:
                    new[(j+xi)*2+k] |= dp[j*2+k]
                if j -xi >= 0:
                    new[(j-xi)*2+(k^1)] |= dp[j*2+k]
            else:
                if j - xi >= 0 and i != 1:
                    new[(j-xi)*2+k] |= dp[j*2+k]
                if j + xi < l*2+50:
                    new[(j+xi)*2+(k^1)] |= dp[j*2+k]
    dp = new
    # print(dp)
mn = INF
for i in range(2*l+50):
    if dp[i*2+1]:
        mn = min(mn, abs(i-c))

print(mn)