use std::io::*;

fn check(a: f64, b: f64, y: f64, z: f64, w: f64) -> bool {
    if (y * y + a * a + w * w - 2. * w * (y * y + a * a).sqrt() - z * z - b * b).abs() < 1e-6 {
        return true;
    }
    false
}

fn main() {
    let mut s: String = String::new();
    std::io::stdin().read_to_string(&mut s).ok();
    let mut itr = s.trim().split_whitespace();
    let x: usize = itr.next().unwrap().parse().unwrap();
    let y: f64 = itr.next().unwrap().parse().unwrap();
    let z: f64 = itr.next().unwrap().parse().unwrap();
    let w: f64 = itr.next().unwrap().parse().unwrap();

    for i in 1..x + 1 {
        if i * i > x {
            break;
        }
        if x % i == 0 {
            let a = i as f64;
            let b = (x / i) as f64;
            if check(a, b, y, z, w) {
                let ans = x as f64 - y * a / 2. - z * b / 2. - (b - y) * (a - z) / 2.;
                println!("{:.10}", ans);
                return;
            }
            if check(b, a, y, z, w) {
                let ans = x as f64 - y * b / 2. - z * a / 2. - (a - y) * (b - z) / 2.;
                println!("{:.10}", ans);
                return;
            }
        }
    }
}