include

using namespace std;

int main() {

int n, m, ans = 0;
cin >> n >> m;
bool f = true;
for (int i = n; i <= m; ++i) {
	f = true;
	for (int j = 2; j * j <= i; j++) {
		if (i % j == 0) {
			f = false;
			break;
		}
		if (f) {
			ans++;
		}
	}
}
cout << ans;

return 0;

}