use std::{collections::BinaryHeap, cmp::Reverse};

const INF: usize = 1usize << 60;
const DX: [isize; 4] = [-1, 1, 0, 0];
const DY: [isize; 4] = [0, 0, -1, 1];

fn main() {
    let mut nm = String::new();
    std::io::stdin().read_line(&mut nm).ok();
    let nm: Vec<usize> = nm.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let n = nm[0];
    let m = nm[1];
    let mut costs = vec![vec![0usize; n]; n];
    for _ in 0..m {
        let mut temp = String::new();
        std::io::stdin().read_line(&mut temp).ok();
        let temp: Vec<usize> = temp.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
        let h = temp[0]-1;
        let w = temp[1]-1;
        let c = temp[2];
        costs[h][w] = c;
    }

    let mut grid = vec![vec![vec![INF; n]; n]; 2];
    grid[0][0][0] = 0;
    let mut pque = BinaryHeap::new();
    pque.push((Reverse(0), 0, 0, 0));
    while let Some((Reverse(c), x, y, layer)) = pque.pop() {
        if c > grid[layer][x][y] { continue; }
        for dir in 0..4 {
            let nx = x as isize + DX[dir];
            let ny = y as isize + DY[dir];
            if nx >= 0 && ny >= 0 && nx < n as isize && ny < n as isize {
                let nx = nx as usize;
                let ny = ny as usize;
                let ncost = grid[layer][x][y] + 1 + costs[nx][ny];
                if grid[layer][nx][ny] > ncost {
                    grid[layer][nx][ny] = ncost;
                    pque.push((Reverse(ncost), nx, ny, layer));
                }
                let ncost = grid[layer][x][y] + 1;
                if costs[nx][ny] > 0 && layer == 0 {
                    if grid[layer][nx][ny] > ncost {
                        grid[layer+1][nx][ny] = ncost;
                        pque.push((Reverse(ncost), nx, ny, layer+1));
                    }
                }
            }
        }
    }
    println!("{}", grid[0][n-1][n-1].min(grid[1][n-1][n-1]));
}