using Lib; using System; using System.Collections.Generic; using System.Diagnostics; using System.IO; using System.Linq; using System.Runtime.CompilerServices; using System.Text; using static Lib.OutputLib; public class Solver { const bool MultiTestCase = false; void Solve() { int n = ri, k = ri; ReadArray(out int[] a, k); var b = new int[2 * k]; for (int i = 0; i < k; i++) b[i] = a[i]; for (int i = 0; i < k; i++) b[i + k] = a[i] + 2 * n; const int LOG = 20; var dp = new int[LOG][]; for (int i = 0; i < LOG; i++) dp[i] = new int[2 * n + 1]; int ok = 0, ng = n + 1; while (ng - ok > 1) { int x = (ok + ng) / 2; for (int i = 0; i < LOG; i++) Array.Fill(dp[i], -1); for (int i = 1, j = 0; i <= n * 2; i++) { while (j + 1 < 2 * k && b[j + 1] <= 2 * i) j++; int p = Math.Min((b[j] - 1) / 2, i - x); if (p < 0) continue; if (j > 0 && p * 2 < b[j - 1]) continue; dp[0][i] = p; for (int l = 0; l + 1 < LOG && dp[l][i] != -1; l++) dp[l + 1][i] = dp[l][dp[l][i]]; } var f = new int[n * 2 + 1]; for (int i = 0; i <= n * 2; i++) f[i] = i; for (int l = 0; l < LOG; l++) { if (((k >> l) & 1) == 0) continue; for (int i = 0; i < f.Length; i++) if (f[i] != -1) f[i] = dp[l][f[i]]; } bool flg = false; for (int i = n; i <= n * 2; i++) flg |= i - n <= f[i]; if (flg) ok = x; else ng = x; } Write(ok * 2); } #pragma warning disable CS0162 public Solver() { if (!MultiTestCase) Solve(); else for (int t = ri; t > 0; t--) Solve(); } #pragma warning restore CS0162 const int IINF = 1 << 30; const long INF = 1L << 60; int ri { [MethodImpl(256)] get => (int)sc.Integer(); } long rl { [MethodImpl(256)] get => sc.Integer(); } uint rui { [MethodImpl(256)] get => (uint)sc.UInteger(); } ulong rul { [MethodImpl(256)] get => sc.UInteger(); } double rd { [MethodImpl(256)] get => sc.Double(); } string rs { [MethodImpl(256)] get => sc.Scan(); } string rline { [MethodImpl(256)] get => sc.Line(); } public StreamScanner sc = new StreamScanner(Console.OpenStandardInput()); void ReadArray(out int[] a, int n) { a = new int[n]; for (int i = 0; i < a.Length; i++) a[i] = ri; } void ReadArray(out long[] a, int n) { a = new long[n]; for (int i = 0; i < a.Length; i++) a[i] = rl; } void ReadArray(out T[] a, int n, Func read) { a = new T[n]; for (int i = 0; i < a.Length; i++) a[i] = read(); } void ReadArray(out T[] a, int n, Func read) { a = new T[n]; for (int i = 0; i < a.Length; i++) a[i] = read(i); } } static class Program { static public void Main(string[] args) { SourceExpander.Expander.Expand(); Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false }); new Solver(); Console.Out.Flush(); } } #region Expanded by https://github.com/kzrnm/SourceExpander namespace Lib{public partial class StreamScanner{public int[][]ReadUnweightedTree(int n,int origin=1){var gr=new List[n];for(int i=0;i();for(int i=0;i=len){ptr=0;if((len=str.Read(buf,0,1024))<=0){isEof=true;return 0;}}return buf[ptr++];}[MethodImpl(256)]public char Char(){byte b=0;do b=Read();while(b<33||126=33&&b<=126;b=(char)Read())sb.Append(b);return sb.ToString();}[MethodImpl(256)]public long Integer(){long ret=0;byte b=0;var ng=false;do b=Read();while(b!='-'&&(b<'0'||'9'double.Parse(Scan());}} namespace Lib{public static class OutputLib{[MethodImpl(256)]public static void WriteJoin(string s,IEnumerablet)=>Console.WriteLine(string.Join(s,t));[MethodImpl(256)]public static void WriteMat(T[,]a,string sep=" "){int sz1=a.GetLength(0),sz2=a.GetLength(1);var b=new T[sz2];for(int i=0;i(T[][]a,string sep=" "){for(int i=0;i(T[][]a,Funcmap,string sep=" "){for(int i=0;imap(x)));}[MethodImpl(256)]public static void Write(object t)=>Console.WriteLine(t.ToString());[MethodImpl(256)]public static void Write(params object[]arg)=>Console.WriteLine(string.Join(" ",arg.Select(x=>x.ToString())));[MethodImpl(256)]public static void Write(string str)=>Console.WriteLine(str);[MethodImpl(256)]public static void WriteFlush(object t){Console.WriteLine(t.ToString());Console.Out.Flush();}[MethodImpl(256)]public static void WriteError(object t)=>Console.Error.WriteLine(t.ToString());[MethodImpl(256)]public static void Flush()=>Console.Out.Flush();[MethodImpl(256)]public static void YN(bool t)=>Console.WriteLine(t?"YES":"NO");[MethodImpl(256)]public static void Yn(bool t)=>Console.WriteLine(t?"Yes":"No");[MethodImpl(256)]public static void yn(bool t)=>Console.WriteLine(t?"yes":"no");[MethodImpl(256)]public static void DeleteLine()=>Console.Write("\x1b[1A\x1b[2K");[MethodImpl(256)]public static void ProgressBar(long now,long total,int blocks=50){int x=(int)((2*now*blocks+1)/(2*total));Console.Write($"\x1b[G[\x1b[42m{string.Concat(Enumerable.Repeat("_",x))}\x1b[0m{string.Concat(Enumerable.Repeat("_",blocks-x))}] : {now} / {total}");}}} namespace SourceExpander{public class Expander{[Conditional("EXP")]public static void Expand(string inputFilePath=null,string outputFilePath=null,bool ignoreAnyError=true){}public static string ExpandString(string inputFilePath=null,bool ignoreAnyError=true){return "";}}} #endregion Expanded by https://github.com/kzrnm/SourceExpander