fn eratosthenes(n: usize) -> Vec<bool> {
    let mut isprime = vec![true; n + 1];
    isprime[0] = false;
    isprime[1] = false;

    for i in 2..=n {
        if !isprime[i] { continue; }
        for j in 2..=n / i {
            isprime[j * i] = false;
        } 
    }
    isprime
}

fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let mut iter = s.trim().split_whitespace();
    let l: usize = iter.next().unwrap().parse().unwrap();
    let r: usize = iter.next().unwrap().parse().unwrap();
    
    let p = eratosthenes(r * 2);

    let mut ans = 0;
    for i in l..=r {
        if p[i] { ans += 1; }
        if i < r && p[2 * i + 1] { ans += 1; }
    }
    println!("{}", ans);
}