#include <iostream>
#include <cstdio>
#include <vector>
#include <cmath>
#include <cstring>
#include <numeric>
#include <algorithm>
#include <functional>
#include <array>
#include <map>
#include <queue>
#include <limits.h>
#include <set>
#include <stack>
#define REP(i,n) for(int i = 0; n > i; i++)

using namespace std;
typedef vector<int> Ivec;
typedef pair<int, int> pii;
int arr[200000];

void Eratosthenes(int N) {
	for (int i = 0; i < N; i++) {
		arr[i] = 1;
	}
	for (int i = 2; i < sqrt(N); i++) {
		if (arr[i]) {
			for (int j = 0; i * (j + 2) < N; j++) {
				arr[i *(j + 2)] = 0;
			}
		}
	}
}

int ketasum(int n, int base) {//桁の総数　345->3+4+5
	if (n == 0)return 0;
	return ketasum(n - n%(base*10), base*10) + n % (base * 10) / base;
}

int main() {
	int a, b;
	scanf("%d %d", &a, &b);

	Eratosthenes(b+1);

	vector<int> sosu, hash;
	for (int i = max(a, 2); b+1 >= i; i++) {
		if (arr[i]) {
			int n = i;
			while ((n / 10)) {
				n = ketasum(n, 1);
			}
			sosu.push_back(i);
			hash.push_back(n);
		}
	}

	int ans = 0, ansst;
	for (int i = sosu.size() - 1; 0 <= i; i--) {
		array<bool, 10> al = {};
		for (int j = i; 0 <= j; j--) {
			if (al[hash[j]]) {
				if (ans < i - j)ansst = sosu[j+1];
				ans = max(ans, i-j);
				break;
			}
			else {
				al[hash[j]] = 1;
			}
		}
	}

	printf("%d\n", ansst);
	return 0;
}