l,n = map(int,input().split())
X = list(map(int,input().split())) + [l]
dp = [[0]*(l+1) for i in range(2)]
dp[0][X[0]] = 1

for bx,x in zip(X,X[1:]):
    dif = x-bx

    for i in range(l)[::-1]:

        if dp[0][i]:
            dp[0][i+dif] = 1
            dp[1][i] = 1
        
        if dp[1][i]:
            dp[0][i+dif] = 1

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