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

int n, m;
int cnt[10];

bool isprime(int x)
{
	if (x <= 1) return false;
	for (int i = 2; i * i <= x; i++){
		if (x % i == 0) return false;
	}
	return true;
}

int hash_(int x)
{
	stringstream ss;
	ss << x;
	
	string s;
	ss >> s;
	if (s.size() == 1) return s[0] - '0';
	int sum = 0;
	for (char c : s) sum += c - '0';
	return hash_(sum);
}

int main()
{
	cin >> n >> m;
	
	vector<int> ps;
	vector<int> hs;
	for (int i = n; i <= m; i++){
		if (isprime(i)){
			ps.push_back(i);
			hs.push_back(hash_(i));
		}
	}

	int l = 0, r = 0;
	int len = 0, res = 0;
	while (r < ps.size()){
		cnt[hs[r]]++;
		while (cnt[hs[r]] == 2){
			cnt[hs[l++]]--;
		}
		if (len <= r - l + 1){
			res = ps[l];
			len = r - l + 1;
		}
		r++;
	}

	cout << res << endl;
}