#include

using namespace std;

bool zhishu(int n) {//判断素数
	int sum = 0;
	for (int i = 2;i < n;i++) {//这里也可以可以2到根号n,但是要注意sqrt后要加一强转为int型
		if (n % i == 0) {
			return false;
		}
	}
	return true;	
}

int main() {
	int a, b;
	while (cin >> a >> b) {
		if (a > b){//注意a b大小顺序
			int temp = a;
			a = b;
			b = temp;
		}
		int count = 0;
		for (int i = a;i <= b;i++) {//开始遍历
			if (zhishu(i)) {
				count++;
			}
		}
		cout << count << endl;	
	}
	return 0;
}