#include<bits/stdc++.h>
using namespace std;
int m,n,s,o;
int main(){
    cin>>m>>n;
    for(int i=m;i<=n;i++){
    	o=0;
		for(int j=2;j<=sqrt(i);j++){
			if(i%j==0) o++; 
		}
		if(o==0&&i!=1) s++;
	}
	cout<<s;
	return 0;       
}
/**************************************************************
	Problem: 1063
	User: zhangziang
	Language: C++
	Result: Accepted
	Time:19 ms
	Memory:2072 kb
****************************************************************/