public class MostFrequentSubstring {
    public static void main (String[] args) throws IOException {
        CBHT<String,Integer> tabela = new CBHT<String,Integer>(300);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        String word = br.readLine().trim();
        String pom;
        String w;
        /*
        *
        * Vashiot kod tuka....
        *
        */
        for(int i=1; i<=word.length(); i++) {
            for(int j=0; j<=word.length()-i; j++) {

                pom=word.substring(j, j+i);

                if(tabela.search(pom)==null) {
                    tabela.insert(pom, 1);

                }
                else
                {
                int val=tabela.search(pom).element.value;
                val++;
                tabela.insert(pom, val);

                }

            }


        }

        //System.out.println(tabela);
        int max=0;
        String maxs="";
        for(int i=1; i<=word.length(); i++) {
            for(int j=0; j<=word.length()-i; j++) {
                w=word.substring(j, j+i);
                int value = tabela.search(w).element.value;
                if(value>max)
                {
                    max=value;
                    maxs=tabela.search(w).element.key;
                }
                else if (value == max) {
                    if (w.length() > maxs.length())
                    {
                    maxs=w;
                    }
                    else if (w.length() == maxs.length())
                    {
                        if (w.compareTo(maxs) < 0) {
                            maxs = w;
                        }
                    }
                }

            }}

        System.out.println(maxs);


    }
}