#![allow(dead_code)]
#![allow(unused_imports)]
#![allow(unused_mut)]
#![allow(unused_macros)]
#![allow(unused_variables)]
#![allow(non_upper_case_globals)]
#![allow(non_snake_case)]
#![allow(unused_unsafe)]
#![allow(unused_must_use)]
use std::cmp::*;
use std::collections::*;
use std::io::{self, stdout, BufRead, BufWriter, Write};

macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

macro_rules! dd {
    ($($a:expr),* $(,)*) => {
        #[cfg(debug_assertions)]
        eprintln!(concat!($("\x1b[38;5;39m|\x1b[m \x1b[38;5;39m", stringify!($a), " =\x1b[m {:?} "),*, "\x1b[38;5;69m|\x1b[m \x1b[38;5;6m"), $(&$a),*);
    };
}

macro_rules! w {
    ($output:expr, $x:expr) => {
        let _ = writeln!($output, "{}", $x);
    };
}

fn sumr(l: usize, r: usize) -> usize {
    let sm = (l + r) * (r - l + 1) / 2;
    return sm;
}

// fn is_prime(x: usize) -> bool {
//     if x < 2 {
//         return false;
//     }
//     if x == 2 || x == 3 || x == 5 {
//         return true;
//     }
//     if x % 2 == 0 || x % 3 == 0 || x % 5 == 0 {
//         return false;
//     }
//     let mut prime = 7.;
//     let mut step = 4.;
//     while prime <= (x as f64).sqrt() {
//         if x as f64 % prime == 0. {
//             return false;
//         }
//         prime += step;
//         step = 6. - step;
//     }

//     return true;
// }
fn is_prime_miller(n: u64) -> bool {
    if n <= 1 {
        return false;
    } else if n <= 3 {
        return true;
    } else if n % 2 == 0 {
        return false;
    }
    let pow = |r: u64, mut m: u64| -> u64 {
        let mut t = 1u128;
        let mut s = (r % n) as u128;
        let n = n as u128;
        while m > 0 {
            if m & 1 == 1 {
                t = t * s % n;
            }
            s = s * s % n;
            m >>= 1;
        }
        t as u64
    };
    let mut d = n - 1;
    let mut s = 0;
    while d % 2 == 0 {
        d /= 2;
        s += 1;
    }
    const B: [u64; 7] = [2, 325, 9375, 28178, 450775, 9780504, 1795265022];
    for &b in B.iter() {
        let mut a = pow(b, d);
        if a <= 1 {
            continue;
        }
        let mut i = 0;
        while i < s && a != n - 1 {
            i += 1;
            a = (a as u128 * a as u128 % n as u128) as u64;
        }
        if i >= s {
            return false;
        }
    }
    true
}
fn main() {
    let out = stdout();
    let mut out = BufWriter::new(out.lock());
    input!(l: usize, r: usize);

    let mut ans = 0;
    for i in l..=r {
        for j in 0..2 {
            let sm = sumr(i, i + j);
            if is_prime_miller(sm as u64) {
                ans += 1;
            }
        }
    }

    writeln!(out, "{}", ans);
}