namespace AtCoder;

#nullable enable

using System.Numerics;

static class Extensions
{
    public static T[] Repeat<T>(this int time, Func<T> F) => Enumerable.Range(0, time).Select(_ => F()).ToArray();
}

class AtCoder
{
    object? Solve()
    {
        var n = Int();
        var k = Int();
        var az = new int[k];
        {
            for (var i = 0; i < k; i++) az[i] = (Int() - 1) >> 1;
            var dz = new int[k];
            dz[0] = az[0] - az[k - 1] + n;
            for (var i = 1; i < k; i++) dz[i] = az[i] - az[i - 1];
            var dmin = dz.Min();
            var dmi = Array.IndexOf(dz, dmin);
            for (var j = 0; j < k; j++) az[j] = dz[(j + dmi) % k];
        }
        var ans = int.MaxValue;
        for (var s = 0; s < az[0]; s++)
        {
            bool F(int x)
            {
                var p = s;
                for (var i = 0; i < k; i++)
                {
                    var a = az[i];
                    if (p >= a) return false;
                    var l = x;
                    l -= a - p;
                    p = Math.Max(l, 0);
                }
                return p <= x;
            }
            int BinarySearch(int pass, int fail)
            {
                while (Math.Abs(pass - fail) > 1)
                {
                    var middle = (pass + fail) >> 1;
                    if (F(middle)) pass = middle; else fail = middle;
                }
                return pass;
            }
            var sns = BinarySearch(0, n + 1);
            ans = Math.Min(ans, sns);
        }
        return ans * 2;
    }

    public static void Main() => new AtCoder().Run();
    public void Run()
    {
        var res = Solve();
        if (res != null)
        {
            if (res is bool yes) res = yes ? "Yes" : "No";
            sw.WriteLine(res);
        }
        sw.Flush();
    }

    string[] input = Array.Empty<string>();
    int iter = 0;
    readonly StreamWriter sw = new(Console.OpenStandardOutput()) { AutoFlush = false };

    string String()
    {
        while (iter >= input.Length) (input, iter) = (Console.ReadLine()!.Split(' '), 0);
        return input[iter++];
    }
    T Input<T>() where T : IParsable<T> => T.Parse(String(), null);
    int Int() => Input<int>();
    void Out(object? x, string? separator = null)
    {
        separator ??= Environment.NewLine;
        if (x is System.Collections.IEnumerable obj and not string)
        {
            var firstLine = true;
            foreach (var item in obj)
            {
                if (!firstLine) sw.Write(separator);
                firstLine = false;
                sw.Write(item);
            }
        }
        else sw.Write(x);
        sw.WriteLine();
    }
}