import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

class MapEntry<K extends Comparable<K>,E> implements Comparable<K> {

    K key;
    E value;

    public MapEntry (K key, E val) {
        this.key = key;
        this.value = val;
    }

    public int compareTo (K that) {
        @SuppressWarnings("unchecked")
        MapEntry<K,E> other = (MapEntry<K,E>) that;
        return this.key.compareTo(other.key);
    }

    public String toString () {
        return "(" + key + "," + value + ")";
    }
}

class SLLNode<E> {
    protected E element;
    protected SLLNode<E> succ;

    public SLLNode(E elem, SLLNode<E> succ) {
        this.element = elem;
        this.succ = succ;
    }

    @Override
    public String toString() {
        return element.toString();
    }
}

class CBHT<K extends Comparable<K>, E> {

    private SLLNode<MapEntry<K,E>>[] buckets;

    @SuppressWarnings("unchecked")
    public CBHT(int m) {
        buckets = (SLLNode<MapEntry<K,E>>[]) new SLLNode[m];
    }

    private int hash(K key) {
        return Math.abs(key.hashCode()) % buckets.length;
    }

    public SLLNode<MapEntry<K,E>> search(K targetKey) {
        int b = hash(targetKey);
        for (SLLNode<MapEntry<K,E>> curr = buckets[b]; curr != null; curr = curr.succ) {
            if (targetKey.equals(((MapEntry<K, E>) curr.element).key))
                return curr;
        }
        return null;
    }

    public void insert(K key, E val) {      // Insert the entry <key, val> into this CBHT.
        MapEntry<K, E> newEntry = new MapEntry<K, E>(key, val);
        int b = hash(key);
        for (SLLNode<MapEntry<K,E>> curr = buckets[b]; curr != null; curr = curr.succ) {
            if (key.equals(((MapEntry<K, E>) curr.element).key)) {
                curr.element = newEntry;
                return;
            }
        }
        buckets[b] = new SLLNode<MapEntry<K,E>>(newEntry, buckets[b]);
    }

    public void delete(K key) {
        int b = hash(key);
        for (SLLNode<MapEntry<K,E>> pred = null, curr = buckets[b]; curr != null; pred = curr, curr = curr.succ) {
            if (key.equals(((MapEntry<K,E>) curr.element).key)) {
                if (pred == null)
                    buckets[b] = curr.succ;
                else
                    pred.succ = curr.succ;
                return;
            }
        }
    }

    public String toString() {
        String temp = "";
        for (int i = 0; i < buckets.length; i++) {
            temp += i + ":";
            for (SLLNode<MapEntry<K,E>> curr = buckets[i]; curr != null; curr = curr.succ) {
                temp += curr.element.toString() + " ";
            }
            temp += "\n";
        }
        return temp;
    }

}

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));
        int max = 0;
        String maxString = "";
        String word = br.readLine().trim();
        String pom[] = new String[10000];
        int c =0;
        int length = word.length();
        for(int i=0;i<length;i++){
            for(int j=1;j<=length - i;j++){
                if(word.substring(i,i+j)!=null)
                    pom[c++] = word.substring(i,i+j);
            }
        }
        for(int i=0; i<pom.length;i++){
            if(pom[i]!=null){
                if(tabela.search(pom[i])==null){
                    tabela.insert(pom[i], 0);
                }
                else{
                    SLLNode<MapEntry<String, Integer>> node = tabela.search(pom[i]);
                    int val = node.element.value + 1;
                    tabela.delete(pom[i]);
                    tabela.insert(pom[i], val);
                }
            }
            else
                break;
        }
        for(int i=0;i<pom.length;i++){
            if(pom[i]==null) break;
            SLLNode<MapEntry<String, Integer>> node = tabela.search(pom[i]);
            if(node.element.value>max){
                max = node.element.value;
                maxString = pom[i];
            }
            else if(node.element.value == max){
                if(maxString.length()<pom[i].length()){
                    maxString = pom[i];
                }
                else if(maxString.length()==pom[i].length()){
                    for(int k = 0; k<maxString.length();k++){
                        if((int)maxString.charAt(k)>(int)pom[i].charAt(k)){
                            maxString = pom[i];
                        }
                    }
                }
            }
        }
        System.out.print(maxString);
    }
}