include

using namespace std;

int main() {

int n, sum = 0;
cin >> n;
int m;
cin >> m;

for ( int i = n ; i <= m; i++) {

	bool flud = true;

	for (int a = 2; a <= i - 1; a++) {

		if (i % a == 0) {

			flud = false;
			break;
		}
	}

	if (flud == true) {
		sum++;
	}

}

cout << sum;
return 0;

}