//! ----------------------------------------------
//! Framework <https://github.com/vain0x/procon>
//!
//! See the bottom of file for solution.
//! ----------------------------------------------

#![allow(unused_imports)]
#![allow(non_snake_case)]

use std::cell::RefCell;
use std::cmp::{max, min, Ordering};
use std::collections::*;
use std::fmt::{Debug, Display, Formatter, Write as FmtWrite};
use std::io::{stderr, stdin, BufRead, Write};
use std::mem::{replace, swap};
use std::ops::*;
use std::rc::Rc;

/// Print values to standard error if debug mode.
#[allow(unused_macros)]
macro_rules! debug {
    ($($e:expr),*) => {
        #[cfg(debug_assertions)]
        $({
            let (e, mut err) = (stringify!($e), stderr());
            writeln!(err, "\x1B[33m{}\x1B[0m = {:?}", e, $e).unwrap()
        })*
    };
}

/// Read from standard input and parse each word.
/// - `read!(T, U, ..)` parses a line as a tuple of words.
/// - `read![[T]]` parses a line as an array of words.
/// - `read![..; N]` parses `N` lines, using `read!(..)` repeatedly.
#[allow(unused_macros)]
macro_rules! read {
    ([$t:ty] ; $n:expr) =>
        ((0..$n).map(|_| read!([$t])).collect::<Vec<_>>());
    ($($t:ty),+ ; $n:expr) =>
        ((0..$n).map(|_| read!($($t),+)).collect::<Vec<_>>());
    ([$t:ty]) =>
        (rl().split_whitespace().map(|w| w.parse().unwrap()).collect::<Vec<$t>>());
    ($($t:ty),*) => {{
        let buf = rl();
        let mut w = buf.split_whitespace();
        ($(w.next().unwrap().parse::<$t>().unwrap()),*)
    }};
}

/// Read a line from standard input.
#[allow(dead_code)]
fn rl() -> String {
    let mut buf = String::new();
    stdin().read_line(&mut buf).unwrap();

    #[allow(deprecated)]
    buf.trim_right().to_owned()
}

// -----------------------------------------------
// Solution
// -----------------------------------------------

#[derive(PartialEq, Eq, Clone, Debug)]
pub struct Rev<T>(pub T);

impl<T: PartialOrd> PartialOrd for Rev<T> {
    fn partial_cmp(&self, other: &Rev<T>) -> Option<Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl<T: Ord> Ord for Rev<T> {
    fn cmp(&self, other: &Rev<T>) -> Ordering {
        other.0.cmp(&self.0)
    }
}

fn main() {
    let (N, M) = read!(usize, usize);
    let T = read![usize, usize, i64; M];

    let mut b = vec![];
    for &(l, r, _) in &T {
        b.push(l);
        b.push(r);
    }
    b.push(N);
    b.sort();
    b.dedup();

    let mut G = vec![vec![]; N + 1];
    for &(l, r, c) in &T {
        G[l].push((r, c));
    }

    for w in b.windows(2) {
        G[w[1]].push((w[0], 0));
    }

    // dyijkstra
    let mut dist = vec![std::i64::MAX; N + 1];
    let mut q: BinaryHeap<(Rev<i64>, usize)> = BinaryHeap::new();

    dist[1] = 0;
    q.push((Rev(0), 1));
    while let Some((Rev(d), u)) = q.pop() {
        for i in 0..G[u].len() {
            let (v, c) = G[u][i];
            if d + c < dist[v] {
                dist[v] = d + c;
                if v == N {
                    break;
                }
                q.push((Rev(d + c), v));
            }
        }
    }

    if dist[N] == std::i64::MAX {
        println!("-1");
    } else {
        println!("{}", dist[N])
    }
}
