import sys
input = lambda: sys.stdin.readline().strip()

def Map():
  return list(map(int,input().split()))

L,N = Map()
X = Map()
dp = [-1 for i in range(N+1)]
dp[0] = 0
l = L // 2
for i in range(N):
  x = X[i]
  if i == 0:
    if x <= l:
      dp[i+1] = x
      continue
  for j in range(i):
    xx = X[j]
    if j == 0:
      if x <= l:
        dp[i+1] = x
    for k in range(1,j+1):
      if dp[k] == -1:
        continue
      a = dp[k] + (x - xx)
      if a <= l:
        dp[i+1] = max(dp[i+1],a)

Y = []
while len(X):
  x = X.pop()
  Y.append(L-x)
X = Y
dp_ = [-1 for i in range(N+1)]
dp_[0] = 0
l = L // 2
for i in range(N):
  x = X[i]
  if i == 0:
    if x <= l:
      dp_[i+1] = x
      continue
  for j in range(i):
    xx = X[j]
    if j == 0:
      if x <= l:
        dp_[i+1] = x
    for k in range(1,j+1):
      if dp_[k] == -1:
        continue
      a = dp_[k] + (x - xx)
      if a <= l:
        dp_[i+1] = max(dp_[i+1],a)

ans = max(max(dp),max(dp_))
ans = L - 2*ans
print(ans)