#![allow(unused_imports, unused_macros)]

use kyoproio::*;
use std::{
    collections::*,
    io::{self, prelude::*},
    iter, mem,
};

fn run<I: Input, O: Write>(mut kin: I, mut out: O) {
    let (n, m): (u32, u32) = kin.parse();
    let b: Vec<u32> = kin.collect(m as usize);
    let mut e = 0.;
    let mut es = 0.;
    let mut x = n;
    for (i, b) in (0..).zip(b.into_iter().rev()) {
        let e1 = e + (x - b) as f64;
        let e2 = (es + m as f64) / i as f64;
        e = e1.min(e2);
        es += e;
        d!(e1, e2, e);
        x = b;
    }
    e += (x - 1) as f64;
    wln!(out, "{}", e);
}

// -----------------------------------------------------------------------------
fn main() -> io::Result<()> {
    std::thread::Builder::new()
        .stack_size(1 << 26)
        .spawn(|| {
            run(
                Scanner::new(io::stdin().lock()),
                io::BufWriter::new(io::stdout().lock()),
            )
        })?
        .join()
        .unwrap();
    Ok(())
}

#[macro_export]
macro_rules! w {
    ($($arg:tt)*) => { write!($($arg)*).unwrap(); }
}
#[macro_export]
macro_rules! wln {
    ($dst:expr $(, $($arg:tt)*)?) => {{
        writeln!($dst $(, $($arg)*)?).unwrap();
        #[cfg(debug_assertions)]
        $dst.flush().unwrap();
    }}
}
#[macro_export]
macro_rules! w_iter {
    ($dst:expr, $fmt:expr, $iter:expr, $delim:expr) => {{
        let mut first = true;
        for elem in $iter {
            if first {
                w!($dst, $fmt, elem);
                first = false;
            } else {
                w!($dst, concat!($delim, $fmt), elem);
            }
        }
    }};
    ($dst:expr, $fmt:expr, $iter:expr) => {
        w_iter!($dst, $fmt, $iter, " ")
    };
}
#[macro_export]
macro_rules! w_iter_ln {
    ($dst:expr, $($t:tt)*) => {{
        w_iter!($dst, $($t)*);
        wln!($dst);
    }}
}
#[macro_export]
macro_rules! e {
    ($($t:tt)*) => {
        #[cfg(debug_assertions)]
        eprint!($($t)*)
    }
}
#[macro_export]
macro_rules! eln {
    ($($t:tt)*) => {
        #[cfg(debug_assertions)]
        eprintln!($($t)*)
    }
}
#[macro_export]
macro_rules! __tstr {
    ($h:expr $(, $t:expr)+) => { concat!(__tstr!($($t),+), ", ", __tstr!(@)) };
    ($h:expr) => { concat!(__tstr!(), " ",  __tstr!(@)) };
    () => { "\x1B[94m[{}:{}]\x1B[0m" };
    (@) => { "\x1B[1;92m{}\x1B[0m = {:?}" }
}
#[macro_export]
macro_rules! d {
    ($($a:expr),*) => { eln!(__tstr!($($a),*), file!(), line!(), $(stringify!($a), $a),*) };
}

pub mod kyoproio {
    use std::{
        fmt::Display,
        io::{self, prelude::*},
        iter::FromIterator,
        marker::PhantomData,
        mem::{self, MaybeUninit},
        str,
    };

    pub trait Input {
        fn bytes(&mut self) -> &[u8];
        fn str(&mut self) -> &str {
            str::from_utf8(self.bytes()).unwrap()
        }
        fn parse<T: Parse>(&mut self) -> T {
            T::parse(self)
        }
        fn parse_iter<T: Parse>(&mut self) -> ParseIter<T, Self> {
            ParseIter(self, PhantomData)
        }
        fn collect<T: Parse, B: FromIterator<T>>(&mut self, n: usize) -> B {
            self.parse_iter().take(n).collect()
        }
        fn map<T: Parse, U, F: FnMut(T) -> U, B: FromIterator<U>>(&mut self, n: usize, f: F) -> B {
            self.parse_iter().take(n).map(f).collect()
        }
    }
    impl<I: Input> Input for &mut I {
        fn bytes(&mut self) -> &[u8] {
            (**self).bytes()
        }
    }
    pub struct Scanner<R> {
        src: R,
        buf: Vec<u8>,
        pos: usize,
        len: usize,
    }
    impl<R: Read> Scanner<R> {
        pub fn new(src: R) -> Self {
            Self {
                src,
                buf: vec![0; 1 << 16],
                pos: 0,
                len: 0,
            }
        }
        fn read(&mut self) -> usize {
            if self.pos > 0 {
                self.buf.copy_within(self.pos..self.len, 0);
                self.len -= self.pos;
                self.pos = 0;
            } else if self.len >= self.buf.len() {
                self.buf.resize(2 * self.buf.len(), 0);
            }
            let n = self.src.read(&mut self.buf[self.len..]).unwrap();
            self.len += n;
            assert!(self.len <= self.buf.len());
            n
        }
    }
    impl<R: Read> Input for Scanner<R> {
        fn bytes(&mut self) -> &[u8] {
            loop {
                while let Some(d) = unsafe { self.buf.get_unchecked(self.pos..self.len) }
                    .iter()
                    .position(u8::is_ascii_whitespace)
                {
                    let p = self.pos;
                    self.pos += d + 1;
                    if d > 0 {
                        return unsafe { self.buf.get_unchecked(p..p + d) };
                    }
                }
                if self.read() == 0 {
                    let p = self.pos;
                    self.pos = self.len;
                    return unsafe { self.buf.get_unchecked(p..self.len) };
                }
            }
        }
    }
    pub struct ParseIter<'a, T, I: ?Sized>(&'a mut I, PhantomData<*const T>);
    impl<'a, T: Parse, I: Input + ?Sized> Iterator for ParseIter<'a, T, I> {
        type Item = T;
        fn next(&mut self) -> Option<T> {
            Some(self.0.parse())
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            (!0, None)
        }
    }
    pub trait Parse: Sized {
        fn parse<I: Input + ?Sized>(src: &mut I) -> Self;
    }
    impl Parse for Vec<u8> {
        fn parse<I: Input + ?Sized>(src: &mut I) -> Self {
            src.bytes().to_owned()
        }
    }
    macro_rules! from_str {
        ($($T:ty)*) => {$(
            impl Parse for $T {
                fn parse<I: Input + ?Sized>(src: &mut I) -> Self {
                    src.str().parse::<$T>().unwrap()
                }
            }
        )*}
    }
    from_str!(String char bool f32 f64);
    macro_rules! int {
        ($($I:ty: $U:ty)*) => {$(
            impl Parse for $I {
                fn parse<I: Input + ?Sized>(src: &mut I) -> Self {
                    let f = |s: &[u8]| s.iter().fold(0, |x, b| 10 * x + (b & 0xf) as $I);
                    let s = src.bytes();
                    if let Some((&b'-', t)) = s.split_first() { -f(t) } else { f(s) }
                }
            }
            impl Parse for $U {
                fn parse<I: Input + ?Sized>(src: &mut I) -> Self {
                    src.bytes().iter().fold(0, |x, b| 10 * x + (b & 0xf) as $U)
                }
            }
        )*}
    }
    int!(isize:usize i8:u8 i16:u16 i32:u32 i64:u64 i128:u128);
    macro_rules! tuple {
        ($H:ident $($T:ident)*) => {
            impl<$H: Parse, $($T: Parse),*> Parse for ($H, $($T),*) {
                fn parse<I: Input + ?Sized>(src: &mut I) -> Self {
                    ($H::parse(src), $($T::parse(src)),*)
                }
            }
            tuple!($($T)*);
        };
        () => {}
    }
    tuple!(A B C D E F G);
    macro_rules! array {
        ($($N:literal)*) => {$(
            impl<T: Parse> Parse for [T; $N] {
                fn parse<I: Input + ?Sized>(src: &mut I) -> Self {
                    unsafe {
                        let mut arr: [MaybeUninit<T>; $N] = MaybeUninit::uninit().assume_init();
                        for elem in &mut arr {
                            *elem = MaybeUninit::new(src.parse());
                        }
                        mem::transmute_copy(&arr)
                    }
                }
            }
        )*}
    }
    array!(1 2 3 4 5 6 7 8);
}