#include <bits/stdc++.h>
using namespace std;
int main () {
	int L, N;
	cin >> L >> N;
	set<int> mp[111][2];
	std::vector<int> A(N);
	for (auto& a : A) cin >> a;
	mp[0][0].insert(A[0]);
	A.push_back(L);
	int pre = A[0];
	int sum = accumulate(A.begin(), A.end(), 0);
	int ans = abs(L - pre * 2);
	for (int i = 1; i <= N; i ++) {
		int x = A[i];
		int d = x - pre;
		for (int p = 0; p < 2; p ++) {
			for (auto& k : mp[i - 1][p]) {
				for (int s = 0; s < 2; s ++) {
					int v = d + k;
					if (s) v = d - k;
					int q = p ^ s;
					if (v + (L - x) <= -ans) continue;
					mp[i][q].insert(v);
					ans = min({ans, abs(v + (L - x))});
				}
			}
		}
		pre = x;
	}
	for (auto k : mp[N][1]) {
		ans = min(ans, abs(k));
	}
	cout << ans << endl;
}