#include <bits/stdc++.h>
using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int L,R; cin >> L >> R;
    
    int Need = R;
    vector<bool> prime(Need+1,true);
    prime.at(0) = false; prime.at(1) = false;
    for(int i=2; i*i<=Need; i++){
        if(!prime.at(i)) continue;
        for(int k=i*i; k<=Need; k+=i) prime.at(k) = false;
    }
    vector<int> P;
    for(int i=L; i<=R; i++) if(prime.at(i)) P.push_back(i);

    int anslen = 0,ansp = -1;
    vector<bool> already(10);
    queue<pair<int,int>> Q;
    for(auto p : P){
        int num = p;
        while(p >= 10){
            int next = 0;
            while(p) next += p%10,p /= 10;
            p = next;
        }
        while(already.at(p)){
            auto[ign,p2] = Q.front(); Q.pop();
            already.at(p2) = false;
        }
        already.at(p) = true; Q.push({num,p});
        if(Q.size() >= anslen) anslen = Q.size(),ansp = Q.front().first; 
    }
    cout << ansp << endl;
}