L,N=map(int, input().split())
X=list(map(int, input().split()))
A=[]
for i in range(N-1):
  A.append(X[i+1]-X[i])

dp=[0]*(2*10**5+10)
dp[0]=1
for a in A:
  ndp=[0]*(2*10**5+10)
  mod=2*10**5+10
  for i in range(2*10**5+10):
    if dp[i]==1:
      #print(a,i+a,i-a)
      ndp[(i+a)%mod]=1
      ndp[i-a]=1
  dp=ndp
  
ans=10**10
for i in range(-10**5-4,10**5+4):
  if dp[i]==1:
    d=i+X[0]-(L-X[-1])
    ans=min(ans,abs(d))
print(ans)