#include <bits/stdc++.h>
#pragma GCC diagnostic ignored "-Wsign-compare"
#pragma GCC diagnostic ignored "-Wsign-conversion"
#define NDEBUG
#define SHOW(...) static_cast<void>(0)
//!===========================================================!//
//!  dP     dP                          dP                    !//
//!  88     88                          88                    !//
//!  88aaaaa88a .d8888b. .d8888b. .d888b88 .d8888b. 88d888b.  !//
//!  88     88  88ooood8 88'  '88 88'  '88 88ooood8 88'  '88  !//
//!  88     88  88.  ... 88.  .88 88.  .88 88.  ... 88        !//
//!  dP     dP  '88888P' '88888P8 '88888P8 '88888P' dP        !//
//!===========================================================!//
template <typename T, typename A>
std::istream& operator>>(std::istream& is, std::vector<T, A>& v)
{
    for (auto& e : v) { is >> e; }
    return is;
}
template <typename T>
T read()
{
    T v;
    return std::cin >> v, v;
}
template <typename T>
std::vector<T> readVec(const std::size_t l)
{
    std::vector<T> v(l);
    for (auto& e : v) { std::cin >> e; }
    return v;
}
using ld = long double;
using uint = unsigned int;
using ll = long long;
using ull = unsigned long long;
constexpr unsigned int MOD = 1000000007;
template <typename T>
constexpr T INF = std::numeric_limits<T>::max() / 4;
template <typename F>
constexpr F PI = static_cast<F>(3.1415926535897932385);
std::mt19937 mt{std::random_device{}()};
template <typename T>
bool chmin(T& a, const T& b) { return (a > b ? a = b, true : false); }
template <typename T>
bool chmax(T& a, const T& b) { return (a < b ? a = b, true : false); }
template <typename T>
std::vector<T> Vec(const std::size_t n, T v) { return std::vector<T>(n, v); }
template <class... Args>
auto Vec(const std::size_t n, Args... args) { return std::vector<decltype(Vec(args...))>(n, Vec(args...)); }
template <typename T>
constexpr T popCount(const T u)
{
#ifdef __has_builtin
    return u == 0 ? T(0) : (T)__builtin_popcountll(u);
#else
    unsigned long long v = static_cast<unsigned long long>(u);
    return v = (v & 0x5555555555555555ULL) + (v >> 1 & 0x5555555555555555ULL), v = (v & 0x3333333333333333ULL) + (v >> 2 & 0x3333333333333333ULL), v = (v + (v >> 4)) & 0x0F0F0F0F0F0F0F0FULL, static_cast<T>(v * 0x0101010101010101ULL >> 56 & 0x7f);
#endif
}
template <typename T>
constexpr T log2p1(const T u)
{
#ifdef __has_builtin
    return u == 0 ? T(0) : T(64 - __builtin_clzll(u));
#else
    unsigned long long v = static_cast<unsigned long long>(u);
    return v = static_cast<unsigned long long>(v), v |= (v >> 1), v |= (v >> 2), v |= (v >> 4), v |= (v >> 8), v |= (v >> 16), v |= (v >> 32), popCount(v);
#endif
}
template <typename T>
constexpr T clog(const T v) { return v == 0 ? T(0) : log2p1(v - 1); }
template <typename T>
constexpr T msbp1(const T v) { return log2p1(v); }
template <typename T>
constexpr T lsbp1(const T v)
{
#ifdef __has_builtin
    return __builtin_ffsll(v);
#else
    return v == 0 ? T(0) : popCount((v & (-v)) - T(1)) + T(1);
#endif
}
template <typename T>
constexpr bool ispow2(const T v) { return popCount(v) == 1; }
template <typename T>
constexpr T ceil2(const T v) { return v == 0 ? T(1) : T(1) << log2p1(v - 1); }
template <typename T>
constexpr T floor2(const T v) { return v == 0 ? T(0) : T(1) << (log2p1(v) - 1); }
//!============================================!//
//!    8888ba.88ba             oo              !//
//!    88  '8b  '8b                            !//
//!    88   88   88  .d8888b.  dP  88d888b.    !//
//!    88   88   88  88'  '88  88  88'  '88    !//
//!    88   88   88  88.  .88  88  88    88    !//
//!    dP   dP   dP  '88888P8  dP  dP    dP    !//
//!============================================!//
int main()
{
    const ll L = read<ll>();
    const int N = read<int>();
    auto X = readVec<ll>(N);
    auto cwd = [&](const int s, const int t) { return (X[t] - X[s] + L) % L; };
    auto naive = [&](const int s) {
        bool clock = false;
        ll sum = X[s];
        std::set<int> res;
        for (int i = s + 1; i < N; i++) { res.insert(i); }
        for (int p = s; not res.empty(); clock = not clock) {
            if (clock) {
                const int nxt = *res.begin();
                sum += cwd(p, nxt), res.erase(nxt), p = nxt;
            } else {
                const int nxt = *res.rbegin();
                sum += cwd(nxt, p), res.erase(nxt), p = nxt;
            }
        }
        return sum;
    };
    auto tle = [&]() {
        ll ans = 0;
        for (int i = 0; i < N; i++) { chmax(ans, naive(i)); }
        for (auto& e : X) { e = L - e; }
        std::reverse(X.begin(), X.end());
        for (int i = 0; i < N; i++) { chmax(ans, naive(i)); }
        std::cout << ans << std::endl;
    };
    auto fast = [&]() {
        ll ans = X.back(), sum = ans;
        for (int s = N - 1, t = N - 1; s >= 1;) {
            bool hoge = (N - 1 - s) % 2 == 0;
            s--, t = hoge ? t : t - 1;
            sum += -cwd(s, t) + cwd(hoge ? t : (t + 1) % N, s);
            chmax(ans, sum);
            SHOW(s, t, sum);
        }
        return ans;
    };
    auto ac = [&]() {
        ll ans = fast();
        for (auto& e : X) { e = L - e; }
        std::reverse(X.begin(), X.end());
        chmax(ans, fast());
        std::cout << ans << std::endl;
    };
    ac();
    //    tle();
    return 0;
}


Measured memory_bytes: 1792