import sys, time, random
from collections import deque, Counter, defaultdict
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 63 - 1
mod = 998244353

l, n = mi()

x = li()
x.append(l)
x.insert(0, 0)
a = []
for i in range(n + 1):
    a.append(x[i + 1] - x[i])

A = a[0]
B = a[-1]
a = a[1:-1]
l = sum(a)
dp = [[False] * (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 + a[i] <= l:
            dp[i + 1][j + a[i]] |= dp[i][j]
ans = inf

for i in range(l + 1):
    if dp[n - 1][i]:
        ans = min(ans, abs(A + i - B - (l - i)))
print(ans)