fn main() {
    let ref mut buf = Vec::new();
    std::io::Read::read_to_end(&mut std::io::stdin(), buf).ok();
    let mut iter = std::str::from_utf8(buf).unwrap().split_whitespace();

    macro_rules! scan {
        ([$t:tt; $n:expr]) => ((0..$n).map(|_| scan!($t)).collect::<Vec<_>>());
        (($($t:tt),*)) => (($(scan!($t)),*));
        ($t:ty) => (iter.next().unwrap().parse::<$t>().unwrap());
    }

    let (n, m) = scan!((f64, usize));
    let b = scan!([f64; m]);

    let first = b.first().unwrap();
    let last = b.last().unwrap();

    let mut res = f64::MAX;
    let mut sum = m as f64;
    for (k, b) in b.iter().enumerate().rev() {
        if k == 0 {
            res = res.min(last - first);
        } else {
            sum += last - b;
            res = res.min(sum / (m - k) as f64);
        }
    }

    let ans = (first - 1.0) + res + (n - last);
    println!("{}", ans);
}