use std::io::Read;

fn solve(abcd: Vec<i16>) {
    println!("{}", (abcd[1]-abcd[0]+1) * (abcd[3]-abcd[2]+1) - std::cmp::max(0, abcd[1] - abcd[2] + 1));
}

fn main() {
    let mut abcd = String::new();
    std::io::stdin().read_to_string(&mut abcd).ok();
    let abcd: Vec<i16> = abcd.trim().split('\n').next().unwrap().trim().split_whitespace().map(|s| s.parse::<i16>().unwrap()).collect();
    solve(abcd);
}