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

/**************************************************************
	Problem: 1063
	User: zhangziang
	Language: C++
	Result: Accepted
	Time:130 ms
	Memory:2072 kb
****************************************************************/