#include <bits/stdc++.h>
using namespace std;
int main(){
	int n,m,i,s,h=0;
	cin>>m>>n;
	for(;m<=n;m++){
		if(m==1){
			m++;
		}
		s=0;
		for(i=2;i<=sqrt(m);i++){
			if(m%i==0){
				s++;
			}
		}
		if(s==0){
			h++;
		}
	}
	cout<<h;
	
}

/**************************************************************
	Problem: 1063
	User: linmiaoling
	Language: C++
	Result: Accepted
	Time:29 ms
	Memory:2072 kb
****************************************************************/