use std::io;

struct Point(f64, f64);

fn read_point() -> Point {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();

    let mut temp = [0.0, 0.0];
    for (i, token) in input.trim().split(' ').enumerate() {
        temp[i] = token.parse().unwrap();
    }
    Point(temp[0], temp[1])
}

fn abs(x: f64) -> f64 {
    if x >= 0.0 { x } else { -x }
}

fn main() {
    let p = read_point();
    let q = read_point();

    println!("{:.20}", (abs(p.0 - q.0) + abs(p.1 - q.1)) / 2.0);
}