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

fn main() {
    let stdin = io::stdin();
    let mut buf = String::new();
    stdin.read_line(&mut buf).ok();
    let mut it = buf.split_whitespace().map(|n| i64::from_str(n).unwrap());
    let (px, py) = (it.next().unwrap(), it.next().unwrap());

    let mut buf = String::new();
    stdin.read_line(&mut buf).ok();
    let mut it = buf.split_whitespace().map(|n| i64::from_str(n).unwrap());
    let (qx, qy) = (it.next().unwrap(), it.next().unwrap());

    let x = (px - qx).abs() as f64;
    let y = (py - qy).abs() as f64;

    println!("{}", (x + y) / 2_f64);
}