#[allow(unused_imports)]
use std::char::from_digit;
#[allow(unused_imports)]
use std::cmp::{max, min};
#[allow(unused_imports)]
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
#[allow(unused_imports)]
use std::fs::File;
#[allow(unused_imports)]
use std::io::prelude::*;
#[allow(unused_imports)]
use std::io::{stdin, stdout, BufReader, BufWriter, Stdin, Stdout, Write};
#[allow(unused_imports)]
use std::mem::swap;
#[allow(unused_imports)]
use std::str::FromStr;
 
type BoxResult<T> = Result<T, Box<std::error::Error>>;


fn main() {
    let mut scanner = Scanner::new(stdin());
    let line = scanner.read_vector::<usize>().unwrap();
    let (n, m) = (line[0], line[1]);
    let mut edges = vec![];
    for _ in 0..m {
        let line = scanner.read_vector::<usize>().unwrap();
        let (l, r, c) = (line[0] - 1, line[1] - 1, line[2] as i64);
        edges.push((l, r, c));
    }
    edges.sort();
    let mut adj = vec![vec![]; n];
    let mut crt = vec![0];
    for (l, r, c) in edges.into_iter() {
        let index = match crt.binary_search(&l) {
            Ok(i) => crt[i],
            Err(i) => {
                if i == crt.len() {
                    println!("-1");
                    return;
                }
                crt[i]
            },
        };
        adj[index].push((r, c));
        match crt.binary_search(&r) {
            Ok(_) => {},
            Err(i) => crt.insert(i, r),
        }
    }
    if crt[crt.len() - 1] != n - 1 {
        println!("-1");
        return;
    }

    let ans = dijkstra(0, n - 1, n, &adj);
    println!("{}", ans);
}

fn encode(tuple: (usize, i64)) -> (i64, usize) {
    (tuple.1 * (-1), tuple.0)
}

fn decode(tuple: (i64, usize)) -> (usize, i64) {
    (tuple.1, tuple.0 * (-1))
}

fn dijkstra(s: usize, e: usize, n: usize, adjacency_list: &Vec<Vec<(usize, i64)>>) -> i64 {
    let mut used = vec![false; n];
    let mut heap = BinaryHeap::<(i64, usize)>::new();
    heap.push(encode((s, 0)));
    let mut answer = 0;
    while heap.len() > 0 {
        let (node, cost) = decode(heap.pop().unwrap());
        if used[node] {
            continue;
        }
        used[node] = true;
        if node == e {
            answer = cost;
            break;
        }
        for &(neighbor, c) in adjacency_list[node].iter() {
            if used[neighbor] {
                continue;
            }
            heap.push(encode((neighbor, cost + c)));
        }
    }
    return answer;
}

#[allow(dead_code)]
struct Scanner<R> {
    reader: BufReader<R>,
    buffer: String,
}
impl<R> Scanner<R>
where
    R: Read,
{
    #[allow(dead_code)]
    fn new(inner: R) -> Scanner<R> {
        let reader = BufReader::new(inner);
        let buffer = String::new();
        Scanner {
            reader: reader,
            buffer: buffer,
        }
    }
    #[allow(dead_code)]
    fn with_capacity(capacity: usize, inner: R) -> Scanner<R> {
        let reader = BufReader::with_capacity(capacity, inner);
        let buffer = String::new();
        Scanner {
            reader: reader,
            buffer: buffer,
        }
    }
 
    #[allow(dead_code)]
    fn read_line(&mut self) -> BoxResult<String> {
        self.reader.read_line(&mut self.buffer)?;
        let cloned = self.buffer.trim().to_string();
        self.buffer.clear();
        Ok(cloned)
    }
 
    #[allow(dead_code)]
    fn read<T>(&mut self) -> BoxResult<T>
    where
        T: FromStr,
        T::Err: std::fmt::Debug,
        T::Err: std::error::Error,
        T::Err: 'static,
    {
        self.reader.read_line(&mut self.buffer)?;
        let parsed = self.buffer.trim().parse()?;
        self.buffer.clear();
        Ok(parsed)
    }
    #[allow(dead_code)]
    fn read_vector<T>(&mut self) -> BoxResult<Vec<T>>
    where
        T: FromStr,
        T::Err: std::fmt::Debug,
        T::Err: std::error::Error,
        T::Err: 'static,
    {
        self.reader.read_line(&mut self.buffer)?;
        let parsed_vec = self
            .buffer
            .trim()
            .split_whitespace()
            .map(|s| s.parse())
            .collect::<Result<Vec<_>, _>>()?;
        self.buffer.clear();
        Ok(parsed_vec)
    }
    #[allow(dead_code)]
    fn read_matrix<T>(&mut self, lines: usize) -> BoxResult<Vec<Vec<T>>>
    where
        T: FromStr,
        T::Err: std::fmt::Debug,
        T::Err: std::error::Error,
        T::Err: 'static,
    {
        let mut parsed_matrix = vec![];
        for _ in 0..lines {
            self.reader.read_line(&mut self.buffer)?;
            parsed_matrix.push(
                self.buffer
                    .trim()
                    .split_whitespace()
                    .map(|s| s.parse())
                    .collect::<Result<Vec<_>, _>>()?,
            );
            self.buffer.clear();
        }
        Ok(parsed_matrix)
    }
    #[allow(dead_code)]
    fn read_chars(&mut self) -> BoxResult<Vec<char>> {
        self.reader.read_line(&mut self.buffer)?;
        let chars = self.buffer.trim().chars().collect::<Vec<_>>();
        self.buffer.clear();
        Ok(chars)
    }
}