#include<bits/stdc++.h>
using namespace std;
int main()
{
	int a,b,ans;
	cin>>a>>b;
	for(int i=a;i<=b;i++) 
	{
		bool flag=0;
		for(int j=2;j*j<=i;j++)
		if(i%j==0)
	{
		flag=1;
		break;
	}
		if(flag==0)
		ans++;
	}
	cout<<ans;
	return 0;
}