#include <bits/stdc++.h>
using namespace std;
int main(){
	int m,n,h=0,i,j,z;
	cin>>m>>n; //m=1 n=20
	for(i=m;i<=n;i++){
		z=0;
		if(i==1){
			z=1;
		}else{
			for(j=2;j<=sqrt(i);j++){
				if(i%j==0){
					z=1;
					break;
				}
		}
		}
		
		if(z==0){
			h++;
		}
	}
	cout<<h;
	
}


/**************************************************************
	Problem: 1063
	User: 15159808820
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/