using System;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(CommonLetter("daadabdbbcd"));

    }

​

    public static char CommonLetter(String str){

        int[] counters = new int['z' - 'a' + 1];

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

            counters[str[i] - 'a']++;       

        }

        int maxIndex = 0;

        for(int i = 1; i < counters.Length; i++) {

            if(counters[i] > counters[maxIndex]) {

                maxIndex = i;

            }

        }

        return (char)('a' + maxIndex);

    }

}