#include <iostream>
using namespace std;
int main() {
	int a = 0, b = 0, cnt = 0;
	cin >> a >> b;
	for (int j = a; j <= b; j++) {
		bool is = true;
		for (int i = 2; i * i <= j; i++) {
			if (j % i == 0) {
				is = false;
				break;
			}
		}
		if (is)
			cnt++;
	}
	cout << cnt << endl;
	return 0;
}