#include <bits/stdc++.h>
using namespace std;
int main(){
    int n,m,i,s,h=0;
    cin>>m>>n;
    for(;m<=n;m++){
        if(m==1){
            m++;
        }
        s=0;
        for(i=2;i<=sqrt(m);i++){
            if(m%i==0){
                s++;
            }
        }
        if(s==0){
            h++;
        }
    }
    cout<<h;
     
}
/**************************************************************
	Problem: 1063
	User: wengsihan
	Language: C++
	Result: Accepted
	Time:29 ms
	Memory:2072 kb
****************************************************************/