fn main() {
    let mut input = String::new();
    std::io::Read::read_to_string(&mut std::io::stdin(), &mut input).ok();
    let input = input
        .split_whitespace()
        .map(|n| n.parse::<u64>().unwrap())
        .collect::<Vec<_>>();

    let n_ab = input[1] - input[0] + 1;
    let n_cd = input[3] - input[2] + 1;

    let ab = input[0]..=input[1];
    let cd = input[2]..=input[3];
    let n_common = ab.filter(|n| cd.contains(n)).count() as u64;

    println!("{}", n_ab * n_cd - n_common);
}