#include<iostream>
#include <fstream>
#include <stdio.h>
#include <stdlib.h>
#define _USE_MATH_DEFINES
#include <math.h>
#include<string>
#include<vector>
#include<cmath>
#include<stack>
#include<queue>
#include<sstream>
#include<algorithm>
#include<map>
#include<complex>
#include<ctime>
#include<set>
using namespace std;


#define li			long long int
#define rep(i,to)	for(li i=0;i<((li)(to));i++)
#define repp(i,start,to)	for(li i=(li)(start);i<((li)(to));i++)
#define pb			push_back
#define sz(v)		((li)(v).size())
#define bgn(v)		((v).begin())
#define eend(v)		((v).end())
#define allof(v)	(v).begin(), (v).end()
#define dodp(v,n)		memset(v,(li)n,sizeof(v))
#define bit(n)		(1ll<<(li)(n))
#define mp(a,b)		make_pair(a,b)
#define rin	rep(i,n)
#define EPS 1e-10
#define ETOL 1e-8
#define MOD 1000000007
#define F first
#define S second
#define p2(a,b)		cout<<a<<"\t"<<b<<endl
#define p3(a,b,c)		cout<<a<<"\t"<<b<<"\t"<<c<<endl

li prime[200001];
vector<li> primes, primess;

li keta_sum(li x){
	li res=0;
	while(x>0){
		res+=x%10;
		x/=10;
	}
	return res;
}

li hashh(li x){
	while(x>=10)x=keta_sum(x);
	return x;
}

int main(){
	rep(i,200001)prime[i]=true;
	prime[0]=prime[1]=false;
	repp(i,2,3100){
		if(prime[i])
		for(li j=2; i*j<200001; j++){
			prime[i*j]=false;
		}
	}


	li l, r;
	cin>>l>>r;

	rep(i,200001)if(prime[i] && i>=l && i<=r)primess.pb(i);
	rep(i,sz(primess))primes.pb(hashh(primess[i]));

	li maxlen=0;
	li res=-1;
	set<li> use;
	li num[10]={0};
	li n=sz(primes);
	li to=0;

	rep(i,n){
		while(to<n){
			if(num[primes[to]]<1){
				use.insert(primes[to]);
				num[primes[to]]++;
			}
			else break;
			to++;
		}
		//p2(to, i);
		if(to-i>maxlen){
			maxlen=to-i;
			res=primess[i];
		}
		else if(to-i==maxlen)res=max(res, primess[i]);

		if(to==n)break;
		num[primes[to]]++;
		use.insert(primes[to]);
		to++;

		while(sz(use)<to-i){
			if(num[primes[i]]==1){
				use.erase(primes[i]);
			}
			num[primes[i]]--;
			i++;
		}
		i--;
	}

	cout<<res<<endl;



	return 0;
}