import sys
input = sys.stdin.readline
L, N = map(int, input().split())
X = list(map(int, input().split()))
A = []
bk = 0
for i in range(N):
    A.append(X[i]-bk)
    bk = X[i]
A.append(L-bk)
dp = set()
dp.add(0)
for i in range(1, len(A)-1):
    ndp = set()
    for d in dp:
        ndp.add(d)
        ndp.add(d+A[i])
    dp = ndp
a = A[0]
b = A[-1]
ans = L
for d in dp:
    ans = min(ans, abs((a+d) - (b+L-(a+b)-d)))
print(ans)