use std::collections::HashSet;
use std::cmp::min;

fn main() {
    let mut input_line = String::new();
    std::io::stdin().read_line(&mut input_line).unwrap();
    let mut iter = input_line.split_whitespace();
    let l: i64 = iter.next().unwrap().parse().unwrap();
    let n: i64 = iter.next().unwrap().parse().unwrap();

    input_line.clear();
    std::io::stdin().read_line(&mut input_line).unwrap();
    let x: Vec<i64> = vec![0]
        .into_iter()
        .chain(input_line.split_whitespace().map(|s| s.parse().unwrap()))
        .chain(vec![l].into_iter())
        .collect();

    let mut dp = HashSet::new();
    dp.insert((0, 0, 0));

    for i in 1..=n+1 {
        let mut ndp = HashSet::new();
        for &(ind, d, com) in dp.iter() {
            if com == 0 {
                ndp.insert((i, d + x[i as usize] - x[ind as usize], 1));
            } else {
                ndp.insert((i, d - (x[i as usize] - x[ind as usize]), 0));
            }
        }

        if i != n + 1 {
            dp = ndp.union(&dp).cloned().collect();
        } else {
            dp = ndp;
        }
    }

    let mut ans = i64::MAX;
    for &(_, d, com) in dp.iter() {
        if com == 0 {
            ans = min(ans, d.abs());
        }
    }

    println!("{}", ans);
}