Problem:
Score : 600 points

Problem Statement

Snuke's town has a subway system, consisting of N stations and M railway lines. The stations are numbered 1 through N. Each line is operated by a company. Each company has an identification number.

The i-th ( 1 \leq i \leq M ) line connects station p_i and q_i bidirectionally. There is no intermediate station. This line is operated by company c_i.

You can change trains at a station where multiple lines are available.

The fare system used in this subway system is a bit strange. When a passenger only uses lines that are operated by the same company, the fare is 1 yen (the currency of Japan). Whenever a passenger changes to a line that is operated by a different company from the current line, the passenger is charged an additional fare of 1 yen. In a case where a passenger who changed from some company A's line to another company's line changes to company A's line again, the additional fare is incurred again.

Snuke is now at station 1 and wants to travel to station N by subway. Find the minimum required fare.

Constraints

2 \leq N \leq 10^5

0 \leq M \leq 2×10^5

1 \leq p_i \leq N (1 \leq i \leq M)

1 \leq q_i \leq N (1 \leq i \leq M)

1 \leq c_i \leq 10^6 (1 \leq i \leq M)

p_i \neq q_i (1 \leq i \leq M)

Input

The input is given from Standard Input in the following format:

N M
p_1 q_1 c_1
:
p_M q_M c_M

Output

Print the minimum required fare. If it is impossible to get to station N by subway, print -1 instead.

Sample Input 1

3 3
1 2 1
2 3 1
3 1 2

Sample Output 1

1

Use company 1's lines: 1 → 2 → 3. The fare is 1 yen.

Sample Input 2

8 11
1 3 1
1 4 2
2 3 1
2 5 1
3 4 3
3 6 3
3 7 3
4 8 4
5 6 1
6 7 5
7 8 5

Sample Output 2

2

First, use company 1's lines: 1 → 3 → 2 → 5 → 6. Then, use company 5's lines: 6 → 7 → 8. The fare is 2 yen.

Sample Input 3

2 0

Sample Output 3

-1

Solution in Rust (1.15.1) (judge verdict: Accepted)

#[allow(unused_imports)]
use std::cmp::{max, min, Ordering};
#[allow(unused_imports)]
use std::collections::{BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
#[allow(unused_imports)]
use std::io::stdin;
#[allow(unused_imports)]
use std::iter::FromIterator;

mod util {
    use std::fmt::Debug;
    use std::io::stdin;
    use std::str::FromStr;

    #[allow(dead_code)]
    pub fn line() -> String {
        let mut line: String = String::new();
        stdin().read_line(&mut line).unwrap();
        line.trim().to_string()
    }

    #[allow(dead_code)]
    pub fn gets<T: FromStr>() -> Vec<T>
    where
        <T as FromStr>::Err: Debug,
    {
        let mut line: String = String::new();
        stdin().read_line(&mut line).unwrap();
        line.split_whitespace()
            .map(|t| t.parse().unwrap())
            .collect()
    }
}

#[allow(unused_macros)]
macro_rules! get {
    ($t:ty) => {
        {
            let mut line: String = String::new();
            stdin().read_line(&mut line).unwrap();
            line.trim().parse::<$t>().unwrap()
        }
    };
    ($($t:ty),*) => {
        {
            let mut line: String = String::new();
            stdin().read_line(&mut line).unwrap();
            let mut iter = line.split_whitespace();
            (
                $(iter.next().unwrap().parse::<$t>().unwrap(),)*
            )
        }
    };
}

#[derive(Eq, PartialEq, Clone, Debug)]
struct Rev<T>(pub T);

impl<T: PartialOrd> PartialOrd for Rev<T> {
    fn partial_cmp(&self, other: &Rev<T>) -> Option<Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl<T: Ord> Ord for Rev<T> {
    fn cmp(&self, other: &Rev<T>) -> Ordering {
        other.0.cmp(&self.0)
    }
}

#[allow(unused_macros)]
macro_rules! debug {
    ($($a:expr),*) => {
        println!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
    }
}

fn main() {
    let (n, m) = get!(usize, usize);
    let pqc: Vec<(usize, usize, usize)> = (0..m).map(|_| get!(usize, usize, usize)).collect();

    let mut edges = HashMap::new();
    let mut cs = vec![BTreeSet::new(); n];

    for &(p, q, c) in &pqc {
        let p = p - 1;
        let q = q - 1;
        edges.entry((p, c)).or_insert(BTreeSet::new()).insert(q);
        edges.entry((q, c)).or_insert(BTreeSet::new()).insert(p);

        cs[p].insert(c);
        cs[q].insert(c);
    }

    let mut que = BinaryHeap::new();
    let mut costs = HashMap::new();
    que.push(Rev((0, (0, None))));

    while let Some(Rev((cost, (station, company)))) = que.pop() {
        if station == n - 1 {
            println!("{}", cost);
            return;
        }

        if let Some(c) = company {
            if let Some(e) = edges.get(&(station, c)) {
                for &to in e.iter() {
                    if costs
                        .get(&(to, company))
                        .cloned()
                        .unwrap_or(u64::max_value()) > cost
                    {
                        costs.insert((to, company), cost);
                        que.push(Rev((cost, (to, company))));
                    }
                }
            }
            let out = (station, None);
            if costs.get(&out).cloned().unwrap_or(u64::max_value()) > cost {
                costs.insert(out, cost);
                que.push(Rev((cost, out)));
            }
        } else {
            for &c in &cs[station] {
                let go = (station, Some(c));
                if costs.get(&go).cloned().unwrap_or(u64::max_value()) > cost + 1 {
                    costs.insert(go, cost + 1);
                    que.push(Rev((cost + 1, go)));
                }
            }
        }
    }

    println!("-1");
}