#include<bits/stdc++.h>
using namespace std;
int prime[1100001];
int sum;
bool isp[1100001];
bool euler(int n) {
	
}
int main() {
	int x, y;
	scanf("%d%d", &x, &y);
	if (x > y)swap(x, y);
	euler(y);
	for (int i = 1; i <= sum; i++) {
		if (prime[i] > x) {
			printf("%d", sum - i + 1);
			return 0;
		}
	}
	printf("0");
}