#![allow(unused_imports)]
use std::cmp::*;
use std::collections::*;
use std::io::Write;
use std::ops::Bound::*;

#[allow(unused_macros)]
macro_rules! debug {
    ($($e:expr),*) => {
        #[cfg(debug_assertions)]
        $({
            let (e, mut err) = (stringify!($e), std::io::stderr());
            writeln!(err, "{} = {:?}", e, $e).unwrap()
        })*
    };
}

fn main() {
    let v = read_vec::<usize>();
    let (n, m) = (v[0], v[1]);
    let mut cost_dict = HashMap::new();
    for i in 0..m {
        let v = read_vec::<i64>();
        let (h, w, c) = (v[0] as usize - 1, v[1] as usize - 1, v[2]);
        cost_dict.insert((h, w), c);
    }

    let mut dp = vec![vec![vec![std::i64::MAX; n]; n]; 2];
    dp[0][0][0] = 0;
    dp[1][0][0] = 0;
    for h in 0..n {
        for w in 0..n {
            let cur_cost;
            if cost_dict.contains_key(&(h, w)) {
                cur_cost = cost_dict[&(h, w)];
            } else {
                cur_cost = 0;
            }
            for k in 0..2 {
                if h > 0 {
                    dp[k][h][w] = min(dp[k][h][w], dp[k][h - 1][w] + 1 + cur_cost);
                }
                if w > 0 {
                    dp[k][h][w] = min(dp[k][h][w], dp[k][h][w - 1] + 1 + cur_cost);
                }
            }
            if h > 0 {
                dp[1][h][w] = min(dp[1][h][w], dp[0][h - 1][w] + 1);
            }
            if w > 0 {
                dp[1][h][w] = min(dp[1][h][w], dp[0][h][w - 1] + 1);
            }
        }
    }
    println!("{}", dp[1][n - 1][n - 1]);
}

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}