fn main() {
    let mut nm = String::new();
    std::io::stdin().read_line(&mut nm).ok();
    let nm: Vec<usize> = nm.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let n = nm[0];
    let m = nm[1];
    let mut b = String::new();
    std::io::stdin().read_line(&mut b).ok();
    let b: Vec<usize> = b.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();

    if m == 1 {
        println!("{}", n-1);
        return;
    }
    let mut summary = vec![0usize; m];
    for i in (0..m-1).rev() {
        summary[i] = summary[i+1] + b[i+1] - b[i];
    }
    let result = n-b[m-1]+b[0]-1;
    let diffmin = b[m-1] - b[0];
    let mut diffmin = diffmin as f64;
    let mut distsum = 0usize;
    for i in (1..m).rev() {
        distsum += summary[i];
        let restdist = distsum as f64 / (m-i) as f64;
        let goalexp = m as f64 / (m-i) as f64;
        let exp = restdist + goalexp;
        if exp < diffmin {
            diffmin = exp;
        }
    }
    println!("{:.09}", result as f64 + diffmin);
}