#![allow(unused_parens)]
#![allow(unused_imports)]
#![allow(non_upper_case_globals)]
#![allow(non_snake_case)]
#![allow(unused_mut)]
#![allow(unused_variables)]
#![allow(dead_code)]

use itertools::Itertools;
use proconio::input;
use proconio::marker::{Chars, Usize1};

#[allow(unused_macros)]
#[cfg(debug_assertions)]
macro_rules! mydbg {
    //($arg:expr) => (dbg!($arg))
    //($arg:expr) => (println!("{:?}",$arg));
      ($($a:expr),*) => {
          eprintln!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
      }
}
#[cfg(not(debug_assertions))]
macro_rules! mydbg {
    ($($arg:expr),*) => {};
}

macro_rules! echo {
    ($($a:expr),*) => {
          $(println!("{}",$a))*
      }
}

use std::cmp::*;
use std::collections::*;
use std::ops::{Add, Div, Mul, Sub};

#[allow(dead_code)]
static INF_I64: i64 = 92233720368547758;
#[allow(dead_code)]
static INF_I32: i32 = 21474836;
#[allow(dead_code)]
static INF_USIZE: usize = 18446744073709551;
#[allow(dead_code)]
static M_O_D: usize = 1000000007;
#[allow(dead_code)]
static PAI: f64 = 3.1415926535897932;

trait IteratorExt: Iterator {
    fn toVec(self) -> Vec<Self::Item>;
}

impl<T: Iterator> IteratorExt for T {
    fn toVec(self) -> Vec<Self::Item> {
        self.collect()
    }
}

trait CharExt {
    fn toNum(&self) -> usize;
    fn toAlphabetIndex(&self) -> usize;
    fn toNumIndex(&self) -> usize;
}
impl CharExt for char {
    fn toNum(&self) -> usize {
        return *self as usize;
    }
    fn toAlphabetIndex(&self) -> usize {
        return self.toNum() - 'a' as usize;
    }
    fn toNumIndex(&self) -> usize {
        return self.toNum() - '0' as usize;
    }
}

trait VectorExt {
    fn joinToString(&self, s: &str) -> String;
}
impl<T: ToString> VectorExt for Vec<T> {
    fn joinToString(&self, s: &str) -> String {
        return self
            .iter()
            .map(|x| x.to_string())
            .collect::<Vec<_>>()
            .join(s);
    }
}

trait StringExt {
    fn get_reverse(&self) -> String;
}
impl StringExt for String {
    fn get_reverse(&self) -> String {
        self.chars().rev().collect::<String>()
    }
}

trait UsizeExt {
    fn pow(&self, n: usize) -> usize;
}
impl UsizeExt for usize {
    fn pow(&self, n: usize) -> usize {
        return ((*self as u64).pow(n as u32)) as usize;
    }
}

pub struct Dikstra {
    dist: Vec<i64>,
    start: usize,
}
impl Dikstra {
    pub fn new(g: &Vec<Vec<(i64, usize)>>, start: usize) -> Self {
        let mut d = Dikstra {
            dist: vec![INF_I64; g.len()],
            start: start,
        };
        d.calc(g);
        return d;
    }
    fn calc(&mut self, g: &Vec<Vec<(i64, usize)>>) {
        let mut used = vec![false; g.len()];
        let mut kouho = std::collections::BinaryHeap::new();
        kouho.push((0, self.start));
        while !kouho.is_empty() {
            let (_, k) = kouho.pop().unwrap();
            used[self.start] = true;
            self.dist[self.start] = 0;
            for i in 0..g[k].len() {
                let (d, p) = g[k][i];
                if used[p] {
                    continue;
                }
                if self.dist[p] > self.dist[k] + d as i64 {
                    self.dist[p] = self.dist[k] + d as i64;
                    kouho.push((self.dist[p] * -1, p));
                }
            }
        }
    }
    pub fn dist_to(&mut self, x: usize) -> i64 {
        mydbg!(self.dist);
        return self.dist[x];
    }
}

fn main() {
    input! {
       N: usize,
       M:usize,
    }

    let mut map = vec![vec![]; N];
    for i in 1..N {
        map[i].push((0, i - 1));
    }
    for _ in 0..M {
        input! {
            u:Usize1,
            v:Usize1,
            c:i64,
        }
        map[u].push((c, v));
    }

    let mut dik = Dikstra::new(&map, 0);
    let ans = dik.dist_to(N - 1);
    if ans == INF_I64 {
        echo!(-1);
    } else {
        echo!(ans);
    }
}


Measured memory_bytes: 13492