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


/**************************************************************
	Problem: 1063
	User: zhangziang
	Language: C++
	Result: Accepted
	Time:12 ms
	Memory:2072 kb
****************************************************************/