#include <iostream>
#include <algorithm>
#include <vector>
#include <cstdio>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)

bool isprime[200000];
//エラトテネス
void eratos(int m){
    for (int i = 0; i <= m; ++i) isprime[i] = true;
    isprime[0] = isprime[1] = false;
    //iを残してiの倍数を消していく
    for (int i = 2; i <= m; ++i){
        if(isprime[i]){
            for (int j = 2 * i; j <= m; j += i){
                isprime[j] = false;
            }
        }
    }
    return;
}

int hashh(int num){
	while(num >= 10){
		int tmp = 0;
		while(num >= 10){
			tmp += num / 10;//1桁ずつ足していく
			num %= 10;
		}
		tmp += num;
		num = tmp;
	}
	return num;
}

int main(void){
	int k, n; cin >> k >> n;
	eratos(n);
	vector<int>	p;
	for (int i = k; i <= n; ++i){
		if(isprime[i]){
			p.push_back(hashh(i));
		}
	}
	
	int ans = 0;
	for (int l = 0; l < p.size(); ++l){
		bool flag[10] = {false};
		int tmp = 0;
		for (int r = l; r < l + 10; ++r){
			if(r >= p.size()) break;
			if(flag[p[r]] == false){
				tmp++; flag[p[r]] = true;
				// printf("r: %d\n", r);
			}
			else{
				// printf("%d %d\n", l, r);
				break;
			}
		}
		ans = max(ans, tmp);
	}
	printf("%d\n", ans);

	return 0;
}