use std::{cmp::Reverse, collections::{BinaryHeap, BTreeSet, HashMap}};

const INF: usize = 1usize << 60;

#[derive(Debug, Clone)]
struct Dijkstra {
    n: usize,
    pathcosts: Vec<Vec<(usize, usize)>>,
    pathcostsrev: Vec<Vec<(usize, usize)>>,
    costs: Vec<usize>
}

impl Dijkstra {
    fn new(n: usize) -> Self {
        Self {
              n: n
            , pathcosts: vec![vec![]; n]
            , pathcostsrev: vec![vec![]; n]
            , costs: vec![INF; n]
        }
    }

    fn pusha2b(&mut self, a: usize, b: usize, cost: usize) {
        self.pathcosts[a].push((cost, b));
        self.pathcostsrev[b].push((cost, a));
    }

    fn get_cost(&self, idx: usize) -> usize {
        self.costs[idx]
    }

    fn solve(&mut self, startpoint: usize) {
        let mut que = BinaryHeap::new();
        que.push(Reverse((0, startpoint)));
        self.costs[startpoint] = 0;
        while let Some(Reverse(p)) = que.pop() {
            let cost = p.0;
            let dest = p.1;
            if cost > self.costs[dest] { continue; }
            for &p2 in self.pathcosts[dest].iter() {
                if self.costs[dest] + p2.0 < self.costs[p2.1] {
                    self.costs[p2.1] = self.costs[dest] + p2.0;
                    que.push(Reverse((self.costs[p2.1], p2.1)));
                }
            }
        }
    }

    fn reconstruct(&self, startpoint: usize, endpoint: usize) -> Vec<(usize, usize)> {
        let mut ret = vec![];
        if self.costs[endpoint] == INF { return ret; }
        let mut current = endpoint;
        while current != startpoint {
            for &(cost, u) in self.pathcostsrev[current].iter() {
                if self.costs[current] == self.costs[u] + cost {
                    let prev = current;
                    current = u;
                    ret.push((current, prev));
                    break;
                }
            }
        }
        ret.reverse();
        ret
    }
}

fn main() {
    let mut nm = String::new();
    std::io::stdin().read_line(&mut nm).ok();
    let nm: Vec<usize> = nm.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let n = nm[0];
    let m = nm[1];
    let mut lines = Vec::with_capacity(m);
    let mut used = BTreeSet::new();
    used.insert(n-1);
    for _ in 0..m {
        let mut ab = String::new();
        std::io::stdin().read_line(&mut ab).ok();
        let ab: Vec<usize> = ab.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
        let a = ab[0];
        let b = ab[1];
        let dist = 2*b - 2*a - 1;
        let a = a-1;
        let b = b-1;
        lines.push((a, b, dist));
        used.insert(a);
        used.insert(b);
    }
    let mut mapping = HashMap::new();
    for (i, &v) in used.iter().enumerate() {
        mapping.insert(v, i);
    }
    let mut paths = Dijkstra::new(used.len());
    for &(u, v, w) in lines.iter() {
        paths.pusha2b(*mapping.get(&u).unwrap(), *mapping.get(&v).unwrap(), w)
    }
    let used = used.into_iter().collect::<Vec<usize>>();
    for i in 0..used.len()-1 {
        let u = used[i];
        let v = used[i+1];
        let dist = (v - u) * 2;
        paths.pusha2b(i, i+1, dist);
    }
    paths.solve(0);
    println!("{}", paths.get_cost(used.len()-1));
}