#![allow(unused_imports, dead_code, unused_macros, unused_variables, non_snake_case, unused_parens)]
use std::cmp::{min,max,Ordering,Reverse};
use std::mem::swap;
use std::collections::{VecDeque,LinkedList,HashMap,BTreeMap,HashSet,BTreeSet,BinaryHeap};

const MOD:u64 = 1_000_000_007;
const INF:i64 = 0x7fff_ffff_ffff_ffff;

macro_rules! min {($a:expr $(,)*) => {{$a}};($a:expr, $b:expr $(,)*) => {{std::cmp::min($a, $b)}};($a:expr, $($rest:expr),+ $(,)*) => {{std::cmp::min($a, min!($($rest),+))}};}
macro_rules! max {($a:expr $(,)*) => {{$a}};($a:expr, $b:expr $(,)*) => {{std::cmp::max($a, $b)}};($a:expr, $($rest:expr),+ $(,)*) => {{std::cmp::max($a, max!($($rest),+))}};}
macro_rules! mulvec {($x:expr; $s:expr) => {vec![$x; $s]};($x:expr; $s0:expr; $( $s:expr );+) => {mulvec![vec![$x; $s0]; $( $s );+ ]};}
trait ChMaxMin<T> {
    fn chmax(&mut self, v: T) -> bool;
    fn chmin(&mut self, v: T) -> bool;
}
impl<T: PartialOrd> ChMaxMin<T> for Option<T> {
    fn chmax(&mut self, v: T) -> bool { if self.is_none() || v > *self.as_ref().unwrap() { *self = Some(v); true } else { false } }
    fn chmin(&mut self, v: T) -> bool { if self.is_none() || v < *self.as_ref().unwrap() { *self = Some(v); true } else { false } }
}
impl<T: PartialOrd> ChMaxMin<T> for T {
    fn chmax(&mut self, v: T) -> bool { if v > *self { *self = v; true } else { false } }
    fn chmin(&mut self, v: T) -> bool { if v < *self { *self = v; true } else { false } }
}
trait Bisect<T> {
    fn bisect_left(&self, x:T) -> usize;
    fn bisect_right(&self, x:T) -> usize;
}
impl <T: PartialOrd> Bisect<T> for Vec<T> {
    fn bisect_left(&self, x:T) -> usize {
        let mut ok = self.len();
        let mut ng = !0;
        while (ok-ng)>1 {
            let m = (ok+ng)/2;
            if self[m] >= x {
                ok = m;
            }else{
                ng = m;
            }
        }
        ok
    }
    fn bisect_right(&self, x:T) -> usize {
        let mut ok = self.len();
        let mut ng = !0;
        while (ok-ng)>1 {
            let m = (ok+ng)/2;
            if self[m] > x {
                ok = m;
            }else{
                ng = m;
            }
        }
        ok
    }
}

fn solve() {
    input! {
        n: usize,
        m: usize,
        A:[usize;m],
    }
    let mut ans = n-m;
    for i in 0..m-1 {
        let mut d = A[i+1]-A[i]-1;
        if d < 3 {continue}
        let mut x = 1;
        while d > 2 {
            if (d & 1) == 1 {
                ans-=x;
                x<<=1;
            }else{
                break;
            }
            d >>= 1;
        }
    }
    println!("{}", ans);
}

fn main() {
    std::thread::Builder::new()
        .stack_size(128 * 1024 * 1024)
        .spawn(|| solve()).unwrap()
        .join().unwrap();
}


mod _input {
    // https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
    #[macro_export]
    macro_rules! input {
        (source = $s:expr, $($r:tt)*) => {
            let mut iter = $s.split_whitespace();
            let mut next = || { iter.next().unwrap() };
            input_inner!{next, $($r)*}
        };
        ($($r:tt)*) => {
            let stdin = std::io::stdin();
            let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
            let mut next = move || -> String{
                bytes
                    .by_ref()
                    .map(|r|r.unwrap() as char)
                    .skip_while(|c|c.is_whitespace())
                    .take_while(|c|!c.is_whitespace())
                    .collect()
            };
            input_inner!{next, $($r)*}
        };
    }
    #[macro_export]
    macro_rules! input_inner {
        ($next:expr) => {};
        ($next:expr, ) => {};

        ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
            let $var = read_value!($next, $t);
            input_inner!{$next $($r)*}
        };
    }
    #[macro_export]
    macro_rules! read_value {
        ($next:expr, ( $($t:tt),* )) => {
            ( $(read_value!($next, $t)),* )
        };

        ($next:expr, [ $t:tt ; $len:expr ]) => {
            (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
        };

        ($next:expr, chars) => {
            read_value!($next, String).chars().collect::<Vec<char>>()
        };

        ($next:expr, usize1) => {
            read_value!($next, usize) - 1
        };

        ($next:expr, $t:ty) => {
            $next().parse::<$t>().expect("Parse error")
        };
    }
}