#include <iostream>
using namespace std;

int a, b;

bool is_prime(int x)
{
	for (int i = 2; i <= x / i; i ++ )
		if (x % i == 0) return false;
	return true;
}

int main()
{
	while (cin >> a >> b)
	{
		int cnt  = 0;
		for (int i = min(a, b); i <= max(a, b); i ++ )
			if (is_prime(i)) cnt ++;
		cout << cnt << endl;
	}
	return 0;
}