using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main(){

        var vals = new []{1,2,2,3,3,3,3,4,7,7,7,8,9,10};

        Console.WriteLine(GetLongestAmp(vals));

    }

    public static int GetLongestAmp(int[] vals)

    {

        var counts = GetCounts(vals);

        var maxbin = new string(Enumerable.Repeat('1',counts.Count()).ToArray());

        return  GetBinaryReps(maxbin)

            .Select(x => FilteredVals(counts, x))

            .Select(x => counts.Where(c => !x.Contains(c.Key)))

            .Select(x => new {vals = x , sum = x.Sum(z => z.Count)})

            .OrderByDescending(x => x.sum).First(x => x.vals.Select(a => a.Key).ToArray().IsAmp()).sum;

    }

    public static int[] FilteredVals(IEnumerable<KeyCount> vals, string switches){

        var output = new List<int>();

        for(int i = 0; i < switches.Length; i++){

            if(switches[i] == '1') output.Add(vals.ElementAt(i).Key);

        }

        return output.ToArray();

    }

    public static IEnumerable<KeyCount> GetCounts(int[] vals){

        return vals.GroupBy(x => x).Select(x => new KeyCount(x.Key,x.Count())).OrderBy(x => x.Count);

    }

    public static IEnumerable<string> GetBinaryReps(string max)

    {

        var maxcount = Convert.ToInt32(max, 2);

        var output = "";

        var count = 0;

        do {

            output = Convert.ToString(count,2).PadLeft(max.Length,'0');

            yield return output;

            count++;

        }while(output != max && count < maxcount + 1);

    }

}

public struct KeyCount{

    public int Key { get; private set; }

    public int Count { get; private set; }

    public KeyCount(int key, int count) : this(){

        Key = key;

        Count = count;

    }

}

public static class Ext{

    public static bool IsAmp(this IEnumerable<int> vals){

        return vals.Count() <= 1 || vals.Max() - vals.Min() <= 1;

    }

}