fn main() {
    let s = {
        let mut s = String::new();
        std::io::stdin().read_line(&mut s).unwrap();
        s.trim_right().to_owned()
    };
    let vec: Vec<i32> = s.split(" ").map(|s| s.parse().unwrap()).collect();
    let (amin, amax) = (vec[0], vec[1]);
    let (bmin, bmax) = (vec[2], vec[3]);
    let mut count = 0;
    for i in amin..(amax+1) {
        for j in bmin..(bmax+1) {
            if i != j {
                count += 1;
            }
        }
    }
    println!("{:?}", count);
}