import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=[0]+list(map(int,input().split()))+[L]

DP={(0,0,0)}

for i in range(1,N+2):
    NDP=set()
    for ind,d,com in DP:
        if com==0:
            NDP.add((i,d+X[i]-X[ind],1))
        else:
            NDP.add((i,d-(X[i]-X[ind]),0))

    if i!=N+1:
        DP=NDP|DP
    else:
        DP=NDP
  
ANS=1<<60
for ind,d,com in DP:
    if com==0:
        ANS=min(ANS,abs(d))

print(ANS)