use std::io::*;
use std::str::FromStr;

#[allow(dead_code)]
fn get_line() -> String {
    let stdin = stdin();
    let mut line = String::new();
    stdin.lock().read_line(&mut line).expect("io error.");
    line.trim().to_string()
}

#[allow(dead_code)]
fn cast<T: FromStr>(s: &str) -> T {
    s.parse().ok().expect("parse error.")
}

#[allow(dead_code)]
fn get_vec<T: FromStr>() -> Vec<T> {
    (&get_line()).split(' ').map(cast::<T>).collect()
}

fn main() {
    let p: Vec<f64> = get_vec();
    let q: Vec<f64> = get_vec();

    let ans: f64 =
        ((p[0] - q[0]).abs() +
         (p[1] - q[1]).abs()) / 2.0;
    println!("{:.1}", ans);
}