#include <bits/stdc++.h>
using namespace std;
int main(){
	int i2,n,sum=0;
	cin>>i2>>n;
	for(int i=i2;i<=n;i++){
		bool bb=1;
		if(i==1)bb=0;
		for(int k=2;k<=sqrt(i);k++){
			if(i%k==0)bb=0;

		}
		if(bb==1)sum++;
		
	}
	cout<<sum;
	return 0;
}
/**************************************************************
	Problem: 1063
	User: zhouhongyi
	Language: C++
	Result: Accepted
	Time:21 ms
	Memory:2072 kb
****************************************************************/