#include <bits/stdc++.h>
using namespace std;
int main(){
  cin.tie(nullptr)->sync_with_stdio(false);
  int L, R; cin >> L >> R;
  vector<int> P, is_P(R + 1, 1);
  is_P[0] = is_P[1] = 0;
  for(long long i = 2; i <= R; ++i){
    if(is_P[i]){
      if(L <= i && i <= R) P.emplace_back(i);
      for(long long j = i * i; j <= R; j += i){
        is_P[j] = 0;
      }
    }
  }

  const int N = P.size();
  vector<int> H(N);
  for(int i = 0; i < N; ++i){
    int x = P[i];
    while(!(x < 10)){
      string s = to_string(x);
      x = 0;
      for(char c: s) x += c - '0';
    }
    H[i] = x;
  }

  int ans = 0, fir = -1;
  vector<int> cnt(10);
  for(int L = 0, R = 0; L < N; ++L){
    while(R < N && cnt[H[R]] == 0) ++cnt[H[R++]];
    if(ans <= R - L){
      ans = R - L;
      fir = P[L];
    }
    cnt[H[L]]--;
  }
  cout << fir << endl;
  return 0;
}