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

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