#![allow(unused_imports)]
use std::cmp::*;
use std::collections::*;
use std::io::Write;
use std::ops::Bound::*;

#[allow(unused_macros)]
macro_rules! debug {
    ($($e:expr),*) => {
        #[cfg(debug_assertions)]
        $({
            let (e, mut err) = (stringify!($e), std::io::stderr());
            writeln!(err, "{} = {:?}", e, $e).unwrap()
        })*
    };
}

fn main() {
    let v = read_vec::<i64>();
    let (n, m) = (v[0], v[1] as usize);
    let b = read_vec::<i64>().iter().map(|&x| x).collect::<Vec<_>>();

    let mut x_min = std::f64::MAX;
    let mut accum = 0.0;
    let mut sum = 0;
    for i in (0..m).rev() {
        accum += 1.0;
        sum += n - b[i];
        x_min = f64::min(x_min, sum as f64 / accum + m as f64 / accum);
    }
    // debug!(x_min);
    let ans = f64::min(n as f64 - 1.0, b[0] as f64 - 1.0 + x_min);
    println!("{}", ans);
}

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}