using System;
using System.Linq;
using System.Collections.Generic;

namespace MyCompiler {
    class Program {
        public static void Main(string[] args) 
        {
            Console.WriteLine("Hello world!");
            maxrepeat("system admin ");
            maxrepeat("Computer Science");
            maxrepeat("the quick brown fox jumps over the lazy dog");
        }
        
        public static void maxrepeat(string str)
        {
            var res = str.Where(x => char.IsLetter(x))
                         .GroupBy(x => x)
                         .ToDictionary(x => x.Key,x => x.Count().ToString()+x.Key.ToString());
            string result = string.Join(", ",res.Values
                                               .ToArray()
                                               .Where(y => y.Contains(res.Values.ToArray().Max()[0]))
                                               .Select(x => x[1]).ToArray());
            Console.WriteLine(result);
        }
    }
}