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

int main()
{
    long long n,k=0,ans=0,b;
	cin>>n>>b;
	if(n==1) {n=2;}
	for(int i=n;i<=b;i++)
	{
		for(int o=2;o<=sqrt(i);o++)
		{
			if(i%o==0)
			{
				k=22;
				break;
			}
	    }
		if(k==0) {ans++;}
		k=0;
	}
	cout<<ans;
	return 0;
}