using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

         var dic = new Dictionary<int, string>();

            for(int i=0; i<2000; i++)

            {

                dic.Add(i, i.ToString());

            }

            var list = dic.Where(f => f.Value.StartsWith("1")).Select(f => f.Key);//.ToList(); //uncomment for fast results 

            Console.WriteLine(list.GetType());

            var list2 = dic.Where(f => list.Contains(f.Key)).ToList();

            Console.WriteLine(list2.Count());

    }

}