using System;
using System.Collections.Generic;

public class Program
{
    public void Proc() {
        Reader.IsDebug = false;
        int min = int.Parse(Reader.ReadLine());
        int max = int.Parse(Reader.ReadLine());
        this.CreatePrimeList(min,max);

        int maxLength = 1;
        int startIdx = 0;
        int ans = -1;
        Dictionary<int, int> hashCount = new Dictionary<int, int>();
        hashCount[HashList[0]] = 1;
        for(int i=0; i<PrimeList.Count; i++) {
            if(startIdx == i) {
                continue;
            }
            int hash = HashList[i];
            if(hashCount.ContainsKey(hash)) {
                for(int j=startIdx; j<i; j++) {
                    hashCount[HashList[j]]--;
                    if(hashCount[HashList[j]] <= 0) {
                        hashCount.Remove(HashList[j]);
                    }
                    if(HashList[j] == hash) {
                        hashCount.Add(hash, 1);
                        startIdx = j + 1;
                        break;
                    }
                }
            } else
            {
                    hashCount.Add(hash, 1);
            }
            if(hashCount.Count >= maxLength) {
                ans = PrimeList[startIdx];
                maxLength = hashCount.Count;
            }
        }
        if(maxLength == 1) {
            ans = PrimeList[PrimeList.Count - 1];
        }

        Console.WriteLine(ans);
    }
    
    private List<int> PrimeList = new List<int>();
    
    private List<int> HashList = new List<int>();
    private void CreatePrimeList(int min, int max) {
        bool[] isProc = new bool[max + 1];
        for(int i=2; i<=max; i++) {
            if (isProc[i] == false) {
                isProc[i] = true;
                if(i >= min) {
                    PrimeList.Add(i);
                    int tmpNum = i;
                    while (tmpNum >= 10)
                    {
                        int total = 0;
                        string strTemp = tmpNum.ToString();
                        foreach (char c in strTemp)
                        {
                            total += int.Parse(c.ToString());
                        }
                        tmpNum = total;
                    }
                    HashList.Add(tmpNum);
                }
                for (int j = 1; j<=max / i; j++) {
                    isProc[i * j] = true;
                }
            }
        }
    }

    public class Reader {
        private static String InitText = @"


10
100


        ";
        
        private static System.IO.StringReader sr = null;
        
        public static bool IsDebug = true;
        
        public static string ReadLine() {
            if(IsDebug) {
                if(sr == null) {
                    sr = new System.IO.StringReader(InitText.Trim());
                }
                return sr.ReadLine();
            } else {
                return Console.ReadLine();
            }
        }
        public static int[] GetInt(char delimiter = ' ') {
            string[] inpt = ReadLine().Split(delimiter);
            int[] ret = new int[inpt.Length];
            for(int i=0; i<inpt.Length; i++) {
                ret[i] = int.Parse(inpt[i]);
            }
            return ret;
        }
    }
    public static void Main(string[] args)
    {
        Program prg = new Program();
        prg.Proc();
    }
}