fn main() {
    let stdin = std::io::read_to_string(std::io::stdin()).unwrap();
    let mut stdin = stdin.split_ascii_whitespace();

    let a: u8 = stdin.next().unwrap().parse().unwrap();
    let b: u8 = stdin.next().unwrap().parse().unwrap();
    let c: u8 = stdin.next().unwrap().parse().unwrap();
    let d: u8 = stdin.next().unwrap().parse().unwrap();

    use std::io::Write;
    let mut out = std::io::BufWriter::new(std::io::stdout().lock());
    out.write_all(output(solve(a, b, c, d)).as_bytes()).unwrap();
}

fn solve(a: u8, b: u8, c: u8, d: u8) -> u16 {
    (b - a + 1) as u16 * (d - c + 1) as u16 - (b.min(d) + 1).saturating_sub(a.max(c)) as u16
}

fn output(ans: u16) -> String {
    ans.to_string() + "\n"
}