#include <bits/stdc++.h>
using namespace std;
int main () {
	int L, N;
	cin >> L >> N;
	unordered_set<int> mp[111][2];
	int pre;
	cin >> pre;
	mp[0][0].insert(pre);
	for (int i = 1; i <= N; i ++) {
		int x;
		if (i < N) cin >> x;
		else x = L;
		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;
					mp[i][q].insert(v);
				}
			}
		}
		pre = x;
	}
	int ans = 1e9 + 7;
	for (auto k : mp[N][1]) {
		ans = min(ans, abs(k));
	}
	cout << ans << endl;
}