#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r) - 1;i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
template <class T, class F>
T binarySearch(T ok, T ng, const F &f) {
	while (abs(ok - ng) > 1) {
		T mid = (ok + ng) / 2;
		(f(mid) ? ok : ng) = mid;
	}
	return ok;
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, k; cin >> n >> k;
	vector<int>a(k);
	rep(i, k)cin >> a[i];
	a.push_back(a[0] + 2 * n);
	rep(i, k) {
		if (a[i + 1] - a[i] < (2 * n) / k) {
			vector<int>na(k);
			rep(j, k) na[j] = (a[(i + j) % k] - a[i] + 2 * n) % (2 * n);
			na.push_back(2 * n);
			swap(a, na);
			break;
		}
	}
	int ans = 0;
	for (int i = a[0] + 1; i < a[1]; i += 2) {
		auto f = [&](int x)->bool {
			int now = i;
			x *= 2;
			rep(i, k - 1) {
				if (now + x >= a[i + 2])return false;
				now = max(now + x, a[i + 1] + 1);
			}
			return now + x <= i + 2 * n;
		};
		auto sub = binarySearch<int>(0, n, f);
		chmax(ans, sub * 2);
	}
	cout << ans << endl;
	return 0;
}