#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int main(){
	int a,b,i,j,x=0,s=0;
	cin>>a>>b;
	for(i=a;i<=b;i++){
		x=0;
		for(j=2;j<=sqrt(i);j++){
			if(i%j==0){
				x++;
			}
		}
		if(x==0&&i!=1)
		s++;
	}cout<<s;
}
/**************************************************************
	Problem: 1063
	User: admin
	Language: C++
	Result: Accepted
	Time:21 ms
	Memory:2072 kb
****************************************************************/