fn main() {
    let stdin = std::io::read_to_string(std::io::stdin()).unwrap();
    let mut stdin = stdin.split_ascii_whitespace();

    let p: (i32, i32) = (
        stdin.next().unwrap().parse().unwrap(),
        stdin.next().unwrap().parse().unwrap(),
    );
    let q: (i32, i32) = (
        stdin.next().unwrap().parse().unwrap(),
        stdin.next().unwrap().parse().unwrap(),
    );

    println!("{}", output(solve(p, q)));
}

fn solve(p: (i32, i32), q: (i32, i32)) -> u32 {
    p.0.abs_diff(q.0) + p.1.abs_diff(q.1)
}

fn output(ans: u32) -> String {
    (ans >> 1).to_string()
        + match ans & 1 {
            0 => "",
            1 => ".5",
            _ => unreachable!(),
        }
}