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

    for i in 2..=(n as f64).sqrt().ceil() as usize {
        if !flgs[i] { continue; }
        for j in i..=n/i {
            flgs[i*j] = false;
        }
    }

    flgs
}

fn main() {
    let mut lr = String::new();
    std::io::stdin().read_line(&mut lr).ok();
    let lr: Vec<usize> = lr.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let l = lr[0];
    let r = lr[1];

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