import sys
input = sys.stdin.buffer.readline

L, N = map(int, input().split())
X = list(map(int, input().split()))
X.append(L)
for i in range(N, 0, -1):
    X[i] -= X[i - 1]

dp0 = 1 << L
dp1 = 0

for x in X:
    dp0 <<= x
    dp1 >>= x
    if N:
        N -= 1
        dp0 = dp1 = dp0 | dp1

ans = bin(dp1)[:1:-1]
print(min(abs(i - L) for i, c in enumerate(ans) if c == '1'))