#![allow(unused_imports, unused_macros)] use kyoproio::*; use std::{ collections::*, io::{self, prelude::*}, iter, mem, }; fn run(mut kin: I, mut out: O) { let (n, m): (u32, u32) = kin.parse(); let b: Vec = 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), $a),*) }; } pub mod kyoproio { use std::{ fmt::Display, io::{self, prelude::*}, iter::FromIterator, marker::PhantomData, mem::{self, MaybeUninit}, str, }; pub trait Input { fn bytes(&mut self) -> &[u8]; fn str(&mut self) -> &str { str::from_utf8(self.bytes()).unwrap() } fn parse(&mut self) -> T { T::parse(self) } fn parse_iter(&mut self) -> ParseIter { ParseIter(self, PhantomData) } fn collect>(&mut self, n: usize) -> B { self.parse_iter().take(n).collect() } fn map U, B: FromIterator>(&mut self, n: usize, f: F) -> B { self.parse_iter().take(n).map(f).collect() } } impl Input for &mut I { fn bytes(&mut self) -> &[u8] { (**self).bytes() } } pub struct Scanner { src: R, buf: Vec, pos: usize, len: usize, } impl Scanner { pub fn new(src: R) -> Self { Self { src, buf: vec![0; 1 << 16], pos: 0, len: 0, } } fn read(&mut self) -> usize { if self.pos > 0 { self.buf.copy_within(self.pos..self.len, 0); self.len -= self.pos; self.pos = 0; } else if self.len >= self.buf.len() { self.buf.resize(2 * self.buf.len(), 0); } let n = self.src.read(&mut self.buf[self.len..]).unwrap(); self.len += n; assert!(self.len <= self.buf.len()); n } } impl Input for Scanner { fn bytes(&mut self) -> &[u8] { loop { while let Some(d) = unsafe { self.buf.get_unchecked(self.pos..self.len) } .iter() .position(u8::is_ascii_whitespace) { let p = self.pos; self.pos += d + 1; if d > 0 { return unsafe { self.buf.get_unchecked(p..p + d) }; } } if self.read() == 0 { let p = self.pos; self.pos = self.len; return unsafe { self.buf.get_unchecked(p..self.len) }; } } } } pub struct ParseIter<'a, T, I: ?Sized>(&'a mut I, PhantomData<*const T>); impl<'a, T: Parse, I: Input + ?Sized> Iterator for ParseIter<'a, T, I> { type Item = T; fn next(&mut self) -> Option { Some(self.0.parse()) } fn size_hint(&self) -> (usize, Option) { (!0, None) } } pub trait Parse: Sized { fn parse(src: &mut I) -> Self; } impl Parse for Vec { fn parse(src: &mut I) -> Self { src.bytes().to_owned() } } macro_rules! from_str { ($($T:ty)*) => {$( impl Parse for $T { fn parse(src: &mut I) -> Self { src.str().parse::<$T>().unwrap() } } )*} } from_str!(String char bool f32 f64); macro_rules! int { ($($I:ty: $U:ty)*) => {$( impl Parse for $I { fn parse(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(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(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 Parse for [T; $N] { fn parse(src: &mut I) -> Self { unsafe { let mut arr: [MaybeUninit; $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); }