pub mod scanner {

    pub struct Scanner {
        buf: Vec<String>,
    }

    impl Scanner {
        pub fn new() -> Self {
            Self { buf: vec![] }
        }

        pub fn new_from(source: &str) -> Self {
            let source = String::from(source);
            let buf = Self::split(source);
            Self { buf }
        }

        pub fn next<T: std::str::FromStr>(&mut self) -> T {
            loop {
                if let Some(x) = self.buf.pop() {
                    return x.parse().ok().expect("");
                }
                let mut source = String::new();
                std::io::stdin().read_line(&mut source).expect("");
                self.buf = Self::split(source);
            }
        }

        fn split(source: String) -> Vec<String> {
            source
                .split_whitespace()
                .rev()
                .map(String::from)
                .collect::<Vec<_>>()
        }
    }
}

use crate::scanner::Scanner;
use std::{collections::BTreeMap, io::Write};
fn main() {
    let mut scanner = Scanner::new();
    let out = std::io::stdout();
    let mut out = std::io::BufWriter::new(out.lock());
    let t: usize = 1;
    for _ in 0..t {
        solve(&mut scanner, &mut out);
    }
}
fn solve(scanner: &mut Scanner, out: &mut std::io::BufWriter<std::io::StdoutLock>) {
    let _n = scanner.next::<usize>();
    let m = scanner.next::<usize>();
    let a = (0..m).map(|_| scanner.next::<usize>()).collect::<Vec<_>>();
    fn dfs(memo: &mut BTreeMap<usize, usize>, x: usize) -> usize {
        if let Some(&res) = memo.get(&x) {
            return res;
        }
        if x <= 1 || x % 2 == 0 {
            return x;
        }
        let mut res = 0;
        for x1 in [(x - 1) / 2, x / 2] {
            let res1 = dfs(memo, x1);
            memo.insert(x1, res1);
            res += res1;
        }
        memo.insert(x, res);
        return res;
    }
    let mut ans = 0usize;
    let mut memo = BTreeMap::new();
    memo.insert(0, 0);
    memo.insert(1, 1);
    memo.insert(2, 2);
    for i in 0..m - 1 {
        ans += dfs(&mut memo, a[i + 1] - a[i] - 1);
    }
    writeln!(out, "{}", ans).unwrap();
}