#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}


Int isprime(Int x){
  if(x<=1) return 0;
  for(Int i=2;i*i<=x;i++)
    if(x%i==0) return 0;
  return 1;
}

//INSERT ABOVE HERE
signed main(){
  Int k,n;
  cin>>k>>n;
  vector<Int> v;
  for(Int i=k;i<=n;i++)
    if(isprime(i)) v.emplace_back(i);

  auto calc=
    [](Int x){
      while(x>=10){
        Int res=0;
        string s=to_string(x);
        for(char c:s) res+=c-'0';
        x=res;
      }
      return x;
    };

  Int ans=0,len=0,tmp=0;
  set<Int> ss;
  for(Int i=0,j=0;i<(Int)v.size();){
    while(i<(Int)v.size()){
      if(ss.count(calc(v[i]))) break;
      ss.emplace(calc(v[i++]));
      tmp++;
    }
    //cout<<v[j]<<" "<<tmp<<endl;
    if(len<=tmp) ans=v[j],len=tmp;
    if(i==(Int)v.size()) break;
    while(ss.count(calc(v[i]))){
      ss.erase(calc(v[j++]));
      tmp--;
    }
  }
  cout<<ans<<endl;
  return 0;
}