use std::io::{self, Read};

fn main() {
    let mut input = String::new();
    io::stdin().read_to_string(&mut input).ok();
    let input = input
        .split_whitespace()
        .map(|n| n.parse::<f64>().unwrap())
        .collect::<Vec<_>>();

    if let &[px, py, qx, qy] = &input[..] {
        let answer = ((qx - px).abs() + (qy - py).abs()) / 2f64;

        println!("{}", answer);
    }
}