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 base = n - b[m-1] + b[0] - 1;
    let warps = b[m-1] - b[0];
    let mut warps = warps as f64;
    let mut summary = 0usize;
    for i in 1..m {
        summary += b[m-1] - b[m-i];
        warps = warps.min(summary as f64 / i as f64 + m as f64 / i as f64);
    }
    println!("{:.09}", base as f64 + warps);
}