using System;
using CompLib.Util;

public class Program
{
    private long L;
    private int N;
    private long[] X;

    public void Solve()
    {
        var sc = new Scanner();
        L = sc.NextLong();
        N = sc.NextInt();
        X = new long[N];
        for (int i = 0; i < N; i++)
        {
            X[i] = sc.NextLong();
        }

        // (右|左)にある程度行く 逆に行く 交互
        long ans = Calc();
        // 左右反転
        for (int i = 0; i < N; i++)
        {
            X[i] = L - X[i];
        }

        Array.Reverse(X);

        ans = Math.Max(ans, Calc());
        Console.WriteLine(ans);
    }

    // 最初右に行く ある程度進んで逆に 以降交互 
    long Calc()
    {
        long[] sum1 = new long[N + 1];
        long[] sum2 = new long[N + 1];
        for (int i = 0; i < N; i++)
        {
            sum1[i + 1] = sum1[i] + X[i];
            sum2[i + 1] = sum2[i] + (L - X[i]);
        }

        long max = long.MinValue;
        // X[i]まで行く 移動距離 X[i]
        for (int i = 0; i < N; i++)
        {
            long a = X[i];

            // 残り本数 M=N-i-1
            int m = N - i - 1;

            // 左に進む ceil(M/2)本
            int l = (m + 1) / 2;
            long b = sum2[N] - sum2[N - l];

            // 右に進む floor(M/2)本
            long r = m / 2;
            long c = sum1[i + 1 + r] - sum1[i + 1];

            long dist;
            if (m % 2 == 0)
            {
                // 最後は右
                // 最後に到達した木はi+r
                dist = a * 2 + b * 2 + c * 2 - X[i + r];
            }
            else
            {
                // 左
                // 最後 N - l
                dist = a * 2 + b * 2 + c * 2 - (L - X[N - l]);
            }

            max = Math.Max(max, dist);
        }

        return max;
    }

    public static void Main(string[] args) => new Program().Solve();
}

namespace CompLib.Util
{
    using System;
    using System.Linq;

    class Scanner
    {
        private string[] _line;
        private int _index;
        private const char Separator = ' ';

        public Scanner()
        {
            _line = new string[0];
            _index = 0;
        }

        public string Next()
        {
            if (_index >= _line.Length)
            {
                string s;
                do
                {
                    s = Console.ReadLine();
                } while (s.Length == 0);

                _line = s.Split(Separator);
                _index = 0;
            }

            return _line[_index++];
        }

        public string ReadLine()
        {
            _index = _line.Length;
            return Console.ReadLine();
        }

        public int NextInt() => int.Parse(Next());
        public long NextLong() => long.Parse(Next());
        public double NextDouble() => double.Parse(Next());
        public decimal NextDecimal() => decimal.Parse(Next());
        public char NextChar() => Next()[0];
        public char[] NextCharArray() => Next().ToCharArray();

        public string[] Array()
        {
            string s = Console.ReadLine();
            _line = s.Length == 0 ? new string[0] : s.Split(Separator);
            _index = _line.Length;
            return _line;
        }

        public int[] IntArray() => Array().Select(int.Parse).ToArray();
        public long[] LongArray() => Array().Select(long.Parse).ToArray();
        public double[] DoubleArray() => Array().Select(double.Parse).ToArray();
        public decimal[] DecimalArray() => Array().Select(decimal.Parse).ToArray();
    }
}

Measured memory_bytes: 48148