#include<iostream>

using namespace std;

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

int main() {
	int a, b;
	while (cin >> a >> b) {
		if (a > b) swap(a, b);
		int cnt = 0;
		for (int i = a; i <= b; i ++ ) {
			if (is_prime(i)) cnt ++ ;
		}
		cout << cnt << endl;
	}

	return 0;
}