use proconio::input;

fn main() {
	input! {
		a: usize,
		b: usize,
		c: usize,
		d: usize,
	}

    let mut ans = 0;

    for i in a..=b {
        for j in c..=d {
            if i != j {
                ans += 1;
            }
        }
    }

    println!("{ans}");
}