#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

// need
#include <iostream>
#include <algorithm>
// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>
#include <unordered_map>
#include <unordered_set>
#include <array>
// stream
//#include <istream>
#include <sstream>
//#include <ostream>
#include <fstream>
// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest&...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int w_=0; w_<n; ++w_){MACRO_VEC_ROW_Scan(w_, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest&...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest&...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& R:c)for(auto& w:R)std::cin>>w;
// output
#define OUT(dist) std::cout<<(dist);
#define FOUT(n, dist) std::cout<<std::fixed<<std::setprecision(n)<<(dist);
#define SOUT(n, c, dist) std::cout<<std::setw(n)<<std::setfill(c)<<(dist);
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(w, n) std::cout<<(w + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define SHOW(dist) {std::cerr << #dist << "\t:" << (dist) << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t:";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}
// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(w, a, n) for(int w=(a);w<(n);++w)
#define RFOR(w, a, n) for(int w=(n)-1;w>=(a);--w)
#define REP(w, n) for(int w=0;w<int(n);++w)
#define RREP(w, n) for(int w=int(n)-1;w>=0;--w)
#define FORLL(w, a, n) for(ll w=ll(a);w<ll(n);++w)
#define RFORLL(w, a, n) for(ll w=ll(n)-1;w>=ll(a);--w)
#define REPLL(w, n) for(ll w=0;w<ll(n);++w)
#define RREPLL(w, n) for(ll w=ll(n)-1;w>=0;--w)
#define IN(a, x, b) (a<=x && x<b)
template<class T> inline T CHMAX(T& a, const T b) { return a = (a < b) ? b : a; }
template<class T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
#define EXCEPTION(msg) throw std::string("Exception : " msg " [ in ") + __func__ + " : " + std::to_string(__LINE__) + " lines ]"
#define TRY(cond, msg) try {if (cond) EXCEPTION(msg);}catch (std::string s) {std::cerr << s << std::endl;}
//void CHECKTIME(std::function<void()> f) { auto start = std::chrono::system_clock::now(); f(); auto end = std::chrono::system_clock::now(); auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start)).count(); std::cerr << "[Time:" << res << "ns  (" << res / (1.0e9) << "s)]\n"; }
// test
template<class T> using V = std::vector<T>;
template<class T> using VV = V<V<T>>;

template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
	os << "(" << p.first << ", " << p.second << ")"; return os;
}

#define random_shuffle "USE std::shuffle!";

// type/const
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = 1 << 30;                          // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-6;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;
template<class T, size_t N> void FILL(T(&a)[N], const T& val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T& val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T>& a, const T& val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>>& a, const T& val) { for (auto& b : a) FILL(b, val); }
// ------------>8------------------------------------->8------------

// ------------>8---------- LazySegmentTree ---------->8------------
namespace MonoidOfValue {
	template <class T> class sum {
	public:
		using value_type = T;
		inline static T op(const T& a, const T& b) { return a + b; }
		inline static value_type id() { return T(0); }
	};
	template <class T> class max {
	public:
		using value_type = T;
		inline static T op(const T& a, const T& b) { return (a < b) ? b : a; }
		inline static T id() { return std::numeric_limits<T>::lowest(); }
	};
	template <class T> class min {
	public:
		using value_type = T;
		inline static T op(const T& a, const T& b) { return (a < b) ? a : b; }
		inline static T id() { return std::numeric_limits<T>::max(); }
	};

	class MONOID {
	public:
		struct S { int a, b; };
		using T = S;
		using value_type = T;
		inline static T op(const T& l, const T& r) { return S{ l.a + r.a, l.b + r.b }; }
		inline static T id() { return S{ 0, 0 }; }
	};
}
namespace MonoidOfOperator {
	template <class T> class add {
	public:
		using value_type = T;
		inline static T op(const T& a, const T& b) { return a + b; }
		inline static T id() { return T(0); }
	};
	template <class T> class upd {
	public:
		using value_type = T;
		inline static T op(const T& a, const T& b) { return b; }
		inline static T id() { return T(0); }
	};
}
namespace ApplyOperator {
	template <class T1, class T2> class RSQRAQ {
	public:
		inline static T1 op(const T1& a, const T2& b, int w) { return a + b * w; }
	};
	template <class T1, class T2> class RSQRUQ {
	public:
		inline static T1 op(const T1& a, const T2& b, int w) { return b * w; }
	};
	template <class T1, class T2> class RMQRAQ {
	public:
		inline static T1 op(const T1& a, const T2& b, int w) { return a + b; }
	};
	template <class T1, class T2> class RMQRUQ {
	public:
		inline static T1 op(const T1& a, const T2& b, int w) { return b; }
	};

	template <class T1, class T2> class AGC030 {
	public:
		inline static T1 op(const T1& a, const T2& b, int w) { return T1{ a.a + a.b * b, a.b }; }
	};
}
template<class MonoidOfValue, class MonoidOfOperator, template<class, class> class ApplyOperator>
class LazySegmentTree {
private:
	using ValueType = typename MonoidOfValue::value_type;
	using OperatorType = typename MonoidOfOperator::value_type;
	using Applier = ApplyOperator<ValueType, OperatorType>;
	std::vector<ValueType> seg;
	std::vector<OperatorType> laz;
	std::vector<bool> isUpdated;
	int HEIGHT;
	int ARY_SIZE;

	void push(int k, int w) {
		if (!isUpdated[k]) {
			seg[k] = Applier::op(seg[k], laz[k], w);
			if (w > 1) {
				laz[(k << 1) + 0] = MonoidOfOperator::op(laz[(k << 1) + 0], laz[k]);
				laz[(k << 1) + 1] = MonoidOfOperator::op(laz[(k << 1) + 1], laz[k]);
				isUpdated[(k << 1) + 0] = false;
				isUpdated[(k << 1) + 1] = false;
			}
			isUpdated[k] = true;
			laz[k] = MonoidOfOperator::id();
		}
	}
	void update(int k, int w) {
		push(k, w);
		if (k < ARY_SIZE) {
			push((k << 1) + 0, w >> 1);
			push((k << 1) + 1, w >> 1);
			seg[k] = MonoidOfValue::op(seg[(k << 1) + 0], seg[(k << 1) + 1]);
		}
	}
public:
	LazySegmentTree(int n) {
		init(n);
	}
	LazySegmentTree(const std::vector<ValueType>& a) {
		init(a.size());
		std::copy(std::begin(a), std::end(a), std::begin(seg) + ARY_SIZE);
		for (int k = ARY_SIZE - 1; k > 0; --k) seg[k] = MonoidOfValue::op(seg[(k << 1) + 0], seg[(k << 1) + 1]);
	}
	void init(int n) {
		HEIGHT = 0;
		while ((1 << HEIGHT) < n) ++HEIGHT;
		ARY_SIZE = 1 << HEIGHT;
		seg.resize(ARY_SIZE << 1, MonoidOfValue::id());
		laz.resize(ARY_SIZE << 1, MonoidOfOperator::id());
		isUpdated.resize(ARY_SIZE << 1, true);
	}
	// ## update [l, r) with v
	void update(int l, int r, OperatorType v) {
		l += ARY_SIZE; r += ARY_SIZE;
		int L = l, R = r - 1;
		for (int i = HEIGHT; i >= 0; --i) push(L >> i, 1 << i), push(R >> i, 1 << i);
		while (l < r) {
			if (l & 1) laz[l] = MonoidOfOperator::op(laz[l], v), isUpdated[l] = false, ++l;
			if (r & 1) --r, laz[r] = MonoidOfOperator::op(laz[r], v), isUpdated[r] = false;
			l >>= 1; r >>= 1;
		}
		for (int i = 1; i <= HEIGHT; ++i) {
			push(L >> i, 1 << i), push(R >> i, 1 << i);
			update(L >> i, 1 << i), update(R >> i, 1 << i);
		}
	}
	// ## return SUM/MAX/MIN in [l, r)
	ValueType query(int l, int r) {
		l += ARY_SIZE; r += ARY_SIZE - 1;
		for (int i = HEIGHT; i >= 0; --i) push(l >> i, 1 << i), push(r >> i, 1 << i);
		++r;
		ValueType vl = MonoidOfValue::id(), vr = MonoidOfValue::id();
		int w = 1;
		while (l < r) {
			if (l & 1) update(l, w), vl = MonoidOfValue::op(vl, seg[l]), ++l;
			if (r & 1) --r, update(r, w), vr = MonoidOfValue::op(seg[r], vr);
			l >>= 1; r >>= 1; w <<= 1;
		}
		return MonoidOfValue::op(vl, vr);
	}
};
// ------------>8---------- LazySegmentTree ---------->8------------

int calc(V<int> d) {
	int n = d.size();
	using S = MonoidOfValue::MONOID::S;
	V<S> a(n);
	REP(i, n) a[i] = S{ 0, d[i] };
	LazySegmentTree<MonoidOfValue::MONOID, MonoidOfOperator::add<int>, ApplyOperator::AGC030> lst(a);

	int ans = 0;
	int r = n;
	REP(i, n) {
		int l = n - 1 - i;
		lst.update(0, l, 1);
		lst.update(l, r, -1);
		if (i % 2 == 0) --r;
		lst.update(r, n, 1);

		CHMAX(ans, lst.query(0, n).a);
	}
	return ans;
}

signed main() {
	INIT;

	VAR(int, L, n);
	VEC(int, x, n);
	x.insert(x.begin(), 0);
	x.emplace_back(L);

	V<int> d(n + 1);
	REP(i, n + 1) d[i] = x[i + 1] - x[i];

	int ans = 0;
	CHMAX(ans, calc(d));
	std::reverse(ALL(d));
	CHMAX(ans, calc(d));
	OUT(ans)BR;

	return 0;
}

Measured memory_bytes: 21988