use std::io::Read;
use std::cmp::{min, max};

fn solve(abcd: Vec<i16>) {
    //let dup = min(abcd[1] - abcd[2] + 1, min(abcd[3] - abcd[2] + 1, abcd[1] - abcd[0] + 1));
    let dup = min(abcd[1], abcd[3]) - max(abcd[0], abcd[2]) + 1;
    println!("{}", (abcd[1]-abcd[0]+1) * (abcd[3]-abcd[2]+1) - max(0, dup));
}

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);
}