#include <bits/stdc++.h>
using namespace std;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
	int n, k;
	cin >> n >> k;
	const int c = 2 * n;
	vector<int> s(c + 1);
	for(int i = 0; i < k; i++){
		int v;
		cin >> v;
		v = (v - 1) / 2;
		s[v]++;
		s[v + n]++;
	}
	for(int i = 0; i < c; i++) s[i + 1] += s[i];
	vector<int> to(c + 1);
	to[c] = c;
	auto f = [&](int mid){
		int cur = 0;
		for(int i = 0; i < n; i++){
			cur = max(cur, i + mid);
			while(cur < c && s[cur] <= s[i]) cur++;
			to[i] = cur;
			to[i + n] = min(c, cur + n);
		}
		cur = 0;
		for(int i = 0; i < k; i++) cur = to[cur];
		if (cur == c) return false;
		if (cur <= n) return true;
		cur -= n;
		const int r = cur + n;
		int res = 0;
		while(to[cur] <= r){
			cur = to[cur];
			res++;
		}
		return res >= k;
	};
	int ok = 1, ng = n / k + 1;
	while(ok + 1 < ng){
		int mid = (ok + ng) / 2;
		(f(mid) ? ok : ng) = mid;
	}
	cout << 2 * ok << '\n';
}