include

using namespace std;

int main() {

int n, m, count = 0, flage;
scanf("%d%d", &n, &m);
for (int i = n; i <= m; i++) {
	flage = 0;
	for (int j = 2; j < i; j++) {
		if (i % j == 0) {
			flage++;
			break;
		}
	}
	if (flage == 0)
		count++;
}
printf("%d", count);
return 0;

}