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 Python (3.4.3) (judge verdict: Memory Limit Exceeded)

import sys,heapq
from collections import defaultdict,deque

input = sys.stdin.readline
n,m = map(int,input().split())
#+路線*chg : その路線ホーム
chg = 10**6

edge=defaultdict(set)
used =defaultdict(bool)
for i in range(m):
    p,q,c = map(int,input().split())
    edge[p].add(p+c*chg)
    edge[p+c*chg].add(p)
    edge[p+c*chg].add(q+c*chg)
    edge[q+c*chg].add(p+c*chg)
    edge[q].add(q+c*chg)
    edge[q+c*chg].add(q)
    used[p] = False
    used[q] = False
    used[p+c*chg] = False
    used[q+c*chg] = False

edgelist = deque()
edgelist.append((1,0))
res = float("inf")

while len(edgelist):
    x,cost = edgelist.pop()
    used[x] = True

    if x == n:
        res = cost
        break
    for e in edge[x]:
        if used[e]:
            continue
        #ホーム→改札
        if x <= 10**5 and chg <= e:
            edgelist.appendleft((e,cost+1))
        else:
            edgelist.append((e,cost))
        
if res == float("inf"):
    print(-1)
else:
    print(res)