#include<cmath>
#include<iostream>
using namespace std;

bool sushu(int n)
{
	if (n < 2)
	{
		return false;
	}
	else
	{
		for (int i = 2; i <= sqrt(n); i++)
		{
			if (n % i == 0)
				return false;
		}
	}
	return true;
}

int main()
{
	int a, b;
	while (cin >> a >> b)
	{
		int sum = 0;
		if (a > b)
		{
			int t = a;
			a = b;
			b = t;
		}
		for (int i = a; i <= b; i++)
		{
			if (sushu(i))
			{
				sum++;
			}
		}
		cout << sum << endl;
	}
	return 0;
}