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 result = (n - b[m-1] + b[0] - 1) as f64;
    let mut middle = (b[m-1] - b[0]) as f64;
    let mut summary = 0usize;
    for i in (1..m).rev() {
        summary += b[m-1] - b[i];
        let prob = (m-i) as f64 / m as f64;
        let exp = 1. / prob;
        let cost = exp + summary as f64 / (m-i) as f64;
        if middle > cost {
            middle = cost;
        }
    }
    println!("{}", result + middle);
}