#include <bits/stdc++.h>
#include <atcoder/all>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#define SELECTER(_1,_2,_3,SELECT,...) SELECT
#define rep1(i,n) for(int i=0;i<int(n);++i)
#define rep2(i,a,n) for(int i=int(a);i<int(n);++i)
#define rep(...) SELECTER(__VA_ARGS__,rep2,rep1)(__VA_ARGS__)
#define RSELECTER(_1, _2, _3, RSELECT, ...) RSELECT
#define rrep1(i,n) for(int i=(int)(n)-1;i>=0;--i)
#define rrep2(i,a,n) for(int i=(int)(n)-1;i>=(int)a;--i)
#define rrep(...) RSELECTER(__VA_ARGS__, rrep2, rrep1)(__VA_ARGS__)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define fi first
#define se second
#define PrintR LogOutput
#ifdef _DEBUG
#define Log(...) LogOutput(__VA_ARGS__)
#else
#define Log(...)
#endif
#define M_PI 3.14159265358979323846
using namespace std;
using namespace atcoder;
using ll=long long;
using ld=long double;
using pii=pair<int,int>;
using pll=pair<long long,long long>;
using pdd=pair<long double,long double>;
using tp=tuple<int,int,int>;
using tpll=tuple<ll,ll,ll>;
using veci=vector<int>;
using vecpii=vector<pair<int,int>>;
using vecll=vector<long long>;
using vecpll=vector<pair<long long,long long>>;
using vecpdd=vector<pair<long double,long double>>;
using vecs=vector<string>;
using vecb=vector<bool>;
using vecd=vector<long double>;
using vectp=vector<tp>;
using vectpll=vector<tpll>;
using mint=modint998244353;
using mint10=modint1000000007;
template<typename T, typename S>
istream& operator>>(istream& in, pair<T, S>& a){return in >> a.first >> a.second;}
template<typename T, typename S>
ostream& operator<<(ostream& out, const pair<T, S>& a){return out << a.first << ' ' << a.second;}
ostream& operator<<(ostream& out, const mint& a){return out << a.val();}
ostream& operator<<(ostream& out, const mint10& a){return out << a.val();}
ostream& operator<<(ostream& out, const modint& a){return out << a.val();}
template<typename T>
ostream& operator<<(ostream& out, const vector<T>& d){for(int i = 0 ; i < d.size() ; ++i) out << d[i] << (i == d.size() - 1 ? "" : " "); return out;}
template<typename T, typename S>
pair<T, S> operator+(const pair<T, S>& a, const pair<T, S>& b){return {a.fi + b.fi, a.se + b.se};}
template<typename T, typename S>
pair<T, S> operator-(const pair<T, S>& a, const pair<T, S>& b){return {a.fi - b.fi, a.se - b.se};}
template<class T> inline bool chmax(T& a,T b){if(a<b) {a=b;return true;} return false;}
template<class T> inline bool chmin(T& a,T b){if(a>b) {a=b;return true;} return false;}
bool Judge(int i, int j, int h, int w){return i < 0 || j < 0 || i >= h || j >= w;}
bool PrintA(int i){cout<<(i ? "Yes" : "No")<<endl;return i;}

constexpr ll INF=numeric_limits<ll>::max() >> 2;
constexpr int inf=numeric_limits<int>::max() >> 1;
constexpr ll MOD=998244353;
const int vi[] = {0, 1, 0, -1}, vj[] = {1, 0, -1, 0};

template<typename... Args>
void LogOutput(Args&&... args){
	stringstream ss;
	((ss << args << ' '), ...);
	cout << ss.str().substr(0, ss.str().length() - 1) << endl;
}
template<typename T>
void LogOutput(vector<vector<T>>& data){for(auto d : data) LogOutput(d);}

int main(){
	ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int n, k;cin>>n>>k;
	n *= 2;
	veci a(k);
	rep(i, k) cin>>a[i], a[i]--;
	rep(i, 1, k) a[i] -= a[0];
	a[0] = 0;
	veci cnt(n);
	rep(i, k) cnt[a[i]]++;
	rep(i, 1, n) cnt[i] += cnt[i - 1];
	veci dp(n), dp2(n);
	auto f = [&](int len){
		rep(i, n) dp[i] = -1;
		int p = 0;
		rep(i, n){
			if(i % 2){
				int j = i - len;
				if(j <= 0 || p == 0){
					j += n;
					if(j % 2 == 0) j--;
					if(cnt[j] == k) dp[i] = 0, dp2[i] = j;
				}else{
					if(j % 2 == 0) j--;
					chmin(j, p - 1);
					if(dp[j] != -1 && cnt[i] - cnt[j] == 1) dp[i] = dp[j] + 1, dp2[i] = dp2[j];
				}
			}else if(cnt[i] - cnt[i - 1]) p = i;
		}
		rep(i, n){
			if(dp[i] >= k) return 1;
			if(dp[i] == k - 1 && i <= dp2[i]) return 1;
		}
		return 0;
	};
	int l = 0, r = n;
	while(r - l > 1){
		int mid = (l + r) / 2;
		if(f(mid)) l = mid;
		else r = mid;
	}
	cout<<l<<endl;
}