using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using static System.Math;
using static System.Array;
using static AtCoder.Tool;
namespace AtCoder
{
    class AC
    {
        const int MOD = 1000000007;
        const int INF = int.MaxValue / 2;
        const long SINF = long.MaxValue / 2;
        const double EPS = 1e-8;
        static readonly int[] dI = { 0, 1, 0, -1 };
        static readonly int[] dJ = { 1, 0, -1, 0 };
        //static List<List<int>> G = new List<List<int>>();
        //static List<List<Edge>>G = new List<List<Edge>>();
        //static List<Edge> E = new List<Edge>();
        static void Main(string[] args)
        {
            //var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            //Console.SetOut(sw);
            var cin = new Scanner();
            while (true)
            {
                int n = int.Parse(Console.ReadLine());
                if (n == 0) { return; }
                var w = cin.ReadSplitInt();
                var dp = new long[n, n];
                for (var i = 0; i < n; i++) dp[i, i] = 0;
                for(var i = 0; i < n - 1; i++)
                {
                    dp[i, i + 1] = Abs(w[i + 1] - w[i]) <= 1 ? 2 : 0;
                }
                for (var L = 2; L < n; L++)
                {
                    for(var i = 0; i + L < n; i++)
                    {
                        if (dp[i + 1, i + L - 1] == L - 1)
                        {
                            dp[i, i + L] = Abs(w[i] - w[i + L]) <= 1 ? L + 1 : L - 1;
                        }
                        for (var j = i + 1; j < i + L; j++)
                        {
                            dp[i, i + L] = Max(dp[i, j] + dp[j + 1, i + L], dp[i, i + L]);
                        }
                    }
                }
                Console.WriteLine(dp[0, n - 1]);
            }

            //Console.Out.Flush();
        }
        struct Edge
        {
            public int from;

            public int to;
            public long dist;
            public Edge(int t, long c)
            {
                from = -1;
                to = t;
                dist = c;
            }
            public Edge(int f, int t, long c)
            {
                from = f;
                to = t;
                dist = c;
            }

        }
    }
    public class Scanner
    {
        public int[] ReadSplitInt()
        {
            return ConvertAll(Console.ReadLine().Split(), int.Parse);
        }
        public long[] ReadSplitLong()
        {
            return ConvertAll(Console.ReadLine().Split(), long.Parse);
        }
        public double[] ReadSplit_Double()
        {
            return ConvertAll(Console.ReadLine().Split(), double.Parse);
        }
    }
    public static class Tool
    {
        static public void Initialize<T>(ref T[] array, T initialvalue)
        {
            for (var i = 0; i < array.Length; i++)
            {
                array[i] = initialvalue;
            }
        }
        static public void Swap<T>(ref T a, ref T b)
        {
            T keep = a;
            a = b;
            b = keep;
        }

        static public void Display<T>(T[,] array2d, int n, int m)
        {
            for (var i = 0; i < n; i++)
            {
                for (var j = 0; j < m; j++)
                {
                    Console.Write($"{array2d[i, j]} ");
                }
                Console.WriteLine();
            }
        }

        static public long LPow(int a, int b)
        {
            return (long)Pow(a, b);
        }
    }
}



Measured memory_bytes: 24876