# coding: utf-8
# Your code here!

L,N=map(int,input().split())
X=list(map(int,input().split()))

X=[0]+X+[L]

diff=[X[i+1]-X[i] for i in range(N+1)]

A=diff[0]
B=diff[-1]
diff=diff[1:-1]

dp=[[0 for i in range(L+1)] for j in range(N)]
dp[0][0]=1


for n in range(N-1):
    for l in range(L+1):
        dp[n+1][l]|=dp[n][l]
        if l+diff[n]<=L:
            dp[n+1][l+diff[n]]|=dp[n][l]

sum_diff=sum(diff)
ans=10**18
for l in range(L+1):
    if dp[-1][l]:
        ans=min(ans,abs(A+l-B-(sum_diff-l)))


print(ans)