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

/**************************************************************
	Problem: 1063
	User: zzz
	Language: C++
	Result: Accepted
	Time:10 ms
	Memory:2072 kb
****************************************************************/