fn main() {
    let mut abcd = String::new();
    std::io::stdin().read_line(&mut abcd).ok();
    let abcd: Vec<usize> = abcd.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let a = abcd[0];
    let b = abcd[1];
    let c = abcd[2];
    let d = abcd[3];

    let mut result = 0usize;
    for i in a..=b {
        for j in c..=d {
            if i != j {
                result += 1;
            }
        }
    }
    println!("{}", result);
}