#[macro_export]
macro_rules! setup {
    { mut $input:ident: SplitWhitespace $(,)? } => {
        use std::io::Read;
        let mut buf = String::new();
        std::io::stdin().read_to_string(&mut buf).ok();
        let mut $input = buf.split_whitespace();
    };
}

#[macro_export]
macro_rules! parse_next {
    ($str_iter:expr) => {
        $str_iter.next().unwrap().parse().ok().unwrap()
    };
}

#[allow(unused_imports)]
use std::collections::{HashMap, HashSet, VecDeque};

fn main() {
    setup! { mut input: SplitWhitespace };

    let a: usize = parse_next!(input);
    let b: usize = parse_next!(input);
    let c: usize = parse_next!(input);
    let d: usize = parse_next!(input);

    let ans = (|| {
        let mut count = 0;
        for i in a..=b {
            if c <= i && i <= d {
                count += 1;
            }
        }
        (b - a + 1) * (d - c + 1) - count
    })();

    println!("{}", ans);
}