//! ----------------------------------------------
//! 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
// -----------------------------------------------

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.sort();
    b.dedup();

    let mut next = vec![N; N + 1];
    for w in b.windows(2) {
        next[w[0]] = w[1];
    }
    debug!(next);

    let mut U = T.iter().map(|&(l, r, c)| (c, l, r)).collect::<Vec<_>>();
    U.sort();
    U.reverse();

    let mut cost = vec![std::i64::MAX; N + 1];
    let mut skip = next.clone();
    for &(c, l, r) in &U {
        let mut v = l;
        while v < r {
            cost[v] = c;
            skip[v] = r;
            v = next[v];
        }
    }
    debug!(cost);

    let mut ok = true;
    let mut total = 0_i64;
    let mut v = 1;
    while v < N {
        if cost[v] == std::i64::MAX {
            ok = false;
            break;
        }
        total += cost[v];
        v = skip[v];
    }

    if !ok {
        println!("-1");
        return;
    }

    println!("{}", total)
}
