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

int main(){
    int M = 200010;
    vector<bool> isprime(M, true);
    deque<int> P;
    isprime[0] = false;
    isprime[1] = false;
    for (int p = 2; p*p <= M; p++){
        if (!isprime[p]) continue;
        for (int m = 2*p; m <= M; m += p){
            isprime[m] = false;
        }
    }
    for (int p = 0; p < M; p++){
        if (isprime[p]) P.push_back(p);
    }

    int L, R;
    cin >> L >> R;
    while (!P.empty() && P.front() < L) P.pop_front();
    while (!P.empty() && P.back() > R) P.pop_back();

    vector<int> V;
    for (int x : P){
        while (x >= 10){
            auto s = to_string(x);
            x = 0;
            for (char c : s) x += c-'0';
        }
        V.push_back(x);
    }

    int N = P.size();
    int len = 0, ans = 0;
    for (int l = 0; l < N; l++){
        set<int> st;
        for (int r = l; r < N; r++){
            if (st.count(V[r])) break;
            st.insert(V[r]);
            if (len <= r-l+1) len = r-l+1, ans = P[l];
        }
    }

    cout << ans << endl;
}