#include <bits/stdc++.h>
#define syosu(x) fixed<<setprecision(x)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef pair<double,double> pdd;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<long double> vd;
typedef vector<vd> vvd;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<string> vs;
typedef vector<P> vp;
typedef vector<vp> vvp;
typedef vector<pll> vpll;
typedef pair<int,P> pip;
typedef vector<pip> vip;
const int inf=1<<27;
const ll INF=1ll<<60;
const double pi=acos(-1);
const double eps=1e-18;
const ll mod=1e9+7;
const int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};

const int M=200001;
int l,r,p[M];

int f(int x){
	if(x<10) return x;
	int t=0;
	while(x) t+=x%10,x/=10;
	return f(t);
}

int main(){
	cin>>l>>r;
	fill(p,p+M,1);
	p[0]=p[1]=0;
	for(int i=2;i<M;i++) if(p[i]){
		for(int j=2*i;j<M;j+=i) p[j]=0;
	}
	vi a,b;
	while(l<=r){
		if(p[l]){
			a.push_back(l);
			b.push_back(f(l));
		}
		l++;
	}
	P p={0,0};
	set<int> st;
	int I=0,n=a.size();
	for(int i=0;i<n;i++){
		while(I<n){
			if(st.find(b[I])==st.end()){
				st.insert(b[I]);
				I++;
			}
			else break;
		}
		p=max(p,{st.size(),a[i]});
		st.erase(b[i]);
	}
	cout<<p.second<<endl;
}