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 mut 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;
    let result = result as f64;
    b.reverse();
    let mut summary = vec![0usize; m];
    for i in 0..m-1 { summary[i+1] = summary[i] + b[i] - b[i+1]; }
    let mut sumval = 0usize;
    let mut val = (b[0] - b[m-1]) as f64;
    for i in 1..m {
        let expval = m as f64 / i as f64;
        let expval = sumval as f64 / i as f64 + expval;
        if val > expval {
            val = expval;
        }
        sumval += summary[i];
    }
    println!("{:.09}", result+val);
}