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: Wrong Answer)

from heapq import heappop, heappush
N,M = map(int, input().split())
edges = [[] for _ in range(N)]
cs = []
c_num = 1
for i in range(M):
	p,q,c = map(int, input().split())
	if not c in cs:
		cs += [c]
		c = c_num
		c_num += 1
	edges[p-1] += [[q-1, c]]
	edges[q-1] += [[p-1, c]]
cs.clear()
distance = [[float('inf')]*(c_num+1) for _ in range(N)]
distance[0][0] = 0
min_distance = [float('inf')]*N
min_distance[0] = 0
Q = [[0, 0]]

while Q:
	s_now,c_now = heappop(Q)
	for t, c in edges[s_now]:
		if min_distance[t] > distance[s_now][c_now] + (c_now != c):
			distance[t][c] = distance[s_now][c_now] + (c_now != c)
			min_distance[t] = distance[t][c]
			heappush(Q, [t, c])
print(min_distance[N-1] if min_distance[N-1] != float('inf') else -1)