#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include <functional>
#include <set>
#include <ctime>
#include <random>
#include <chrono>
#include <cassert>
using namespace std;

namespace {
  using Integer = long long; //__int128;
  template<class T> istream& operator >> (istream& is, vector<T>& vec){for(T& val: vec) is >> val; return is;}
  template<class T> istream& operator ,  (istream& is, T& val){ return is >> val;}
  template<class T> ostream& operator << (ostream& os, const vector<T>& vec){for(int i=0; i<vec.size(); i++) os << vec[i] << (i==vec.size()-1?"":" "); return os;}
  template<class T> ostream& operator ,  (ostream& os, const T& val){ return os << " " << val;}

  template<class H> void print(const H& head){ cout << head; }
  template<class H, class ... T> void print(const H& head, const T& ... tail){ cout << head << " "; print(tail...); }
  template<class ... T> void println(const T& ... values){ print(values...); cout << endl; }

  template<class H> void eprint(const H& head){ cerr << head; }
  template<class H, class ... T> void eprint(const H& head, const T& ... tail){ cerr << head << " "; eprint(tail...); }
  template<class ... T> void eprintln(const T& ... values){ eprint(values...); cerr << endl; }

  class range{
    long long start_, end_, step_;
   public:
    struct range_iterator{
      long long val, step_;
      range_iterator(long long v, long long step) : val(v), step_(step) {}
      long long operator * (){return val;}
      void operator ++ (){val += step_;}
      bool operator != (range_iterator& x){return step_ > 0 ? val < x.val : val > x.val;}
    };
    range(long long len) : start_(0), end_(len), step_(1) {}
    range(long long start, long long end) : start_(start), end_(end), step_(1) {}
    range(long long start, long long end, long long step) : start_(start), end_(end), step_(step) {}
    range_iterator begin(){ return range_iterator(start_, step_); }
    range_iterator   end(){ return range_iterator(  end_, step_); }
  };

  string operator "" _s (const char* str, size_t size){ return move(string(str)); }
  constexpr Integer my_pow(Integer x, Integer k, Integer z=1){return k==0 ? z : k==1 ? z*x : (k&1) ? my_pow(x*x,k>>1,z*x) : my_pow(x*x,k>>1,z);}
  constexpr Integer my_pow_mod(Integer x, Integer k, Integer M, Integer z=1){return k==0 ? z%M : k==1 ? z*x%M : (k&1) ? my_pow_mod(x*x%M,k>>1,M,z*x%M) : my_pow_mod(x*x%M,k>>1,M,z);}
  constexpr unsigned long long operator "" _ten (unsigned long long value){ return my_pow(10,value); }

  inline int k_bit(Integer x, int k){return (x>>k)&1;} //0-indexed

  mt19937 mt(chrono::duration_cast<chrono::nanoseconds>(chrono::steady_clock::now().time_since_epoch()).count());

  template<class T> string join(const vector<T>& v, const string& sep){
    stringstream ss; for(int i=0; i<v.size(); i++){ if(i>0) ss << sep; ss << v[i]; } return ss.str();
  }

  string operator * (string s, int k){ string ret; while(k){ if(k&1) ret += s; s += s; k >>= 1; } return ret; }

}


typedef struct{
  int to;
  int cost;
}edge;
 
 //distance from s
//O(E log V)
template<class T = int>
void dijkstra(vector<vector<edge> > &G, vector<T> &dist, int s, const T INF = 100000000){
  
  //first : distance from s, second : its vertex
  priority_queue< pair<T, int> , vector<pair<T, int> >, greater< pair<T, int> > > pq;
   
  fill(dist.begin(), dist.end(), INF);
  dist[s] = 0;
  pq.push( {dist[s], s} );

  while(!pq.empty()){
    auto q = pq.top();
    pq.pop();
    int from = q.second;
    if(dist[from] < q.first) continue; // it's not minimum distance

    for(auto& e: G[from]){
      if(dist[e.to] > dist[from] + e.cost){
        dist[e.to] = dist[from] + e.cost;
        pq.push( make_pair(dist[e.to], e.to) );
      }
    }
  }
}
 
void add_edge(vector<vector<edge> > &G, int from, int to, int cost){
  G[from].push_back( (edge){to, cost} );
  //G[to].push_back( (edge){from, cost} );
}


class UnionFindTree{
  struct base_node{
    int parent;
    int rank;
    int size;
  };
  
  vector<base_node> node;
 public:
  UnionFindTree(int n){
    node.resize(n);
    for(int i=0; i<n; i++){
      node[i].parent=i;
      node[i].rank=0;
      node[i].size=1;
    }
  }

  int find(int x){  //return root node of x
    if(node[x].parent == x) return x;
    else{
      return node[x].parent = find(node[x].parent);
    }
  }
  
  bool same(int x, int y){
    return find(x) == find(y);
  }

  int size(int at){
    return node[find(at)].size;
  }

  void unite(int x, int y){
    x = find(node[x].parent);
    y = find(node[y].parent);

    if(x==y) return;

    if(node[x].rank < node[y].rank){
      node[x].parent = y;
      node[y].size += node[x].size;
    }else if(node[x].rank > node[y].rank){
      node[y].parent = x;
      node[x].size += node[y].size;
    }else{
      node[x].rank++;
      unite(x,y);
    }
  }
};

constexpr long long mod = 9_ten + 7;

int main(){
  int n,m;
  cin >> n,m;
  vector<int> u(m),v(m),c(m);
  for(auto i : range(m) ){
    scanf("%d%d%d", &u[i], &v[i], &c[i]);
    u[i]--;
    v[i]--;
    c[i]--;
  }

  vector<pair<int,int>> p;
  for(auto i : range(m) ){
    p.push_back({u[i], c[i]});
    p.push_back({v[i], c[i]});
  }
  sort(p.begin(), p.end());
  p.erase( unique(p.begin(), p.end()), p.end());

  vector<vector<int>> d(n);
  for(auto i : range(p.size()) ){
    d[p[i].first].push_back(p[i].second);
  }

  vector<vector<edge>> G(p.size() + n + 1);
  for(auto i : range(m) ){
    int x = lower_bound(p.begin(), p.end(), pair<int,int>{u[i], c[i]}) - p.begin();
    int y = lower_bound(p.begin(), p.end(), pair<int,int>{v[i], c[i]}) - p.begin();
    add_edge(G, x, y, 0);
    add_edge(G, y, x, 0);
  }

  for(auto i : range(n) ){
    if(d[i].size() > 1){
      for(auto w : d[i] ){
        int x = lower_bound(p.begin(), p.end(), pair<int,int>{i, w}) - p.begin();
        add_edge(G, p.size()+i, x, 1);
        add_edge(G, x, p.size()+i, 1);
      }
    }
  }

  for(auto i : range(p.size()) ){
    if(p[i].first == 0){
      add_edge(G, G.size()-1, i, 2);
    }
  }

  vector<int> dist(G.size());
  dijkstra(G, dist, G.size()-1);

  int ans = 100000000;
  for(auto i : range(p.size()) ){
    if(p[i].first != n-1) continue;
    ans = min(ans, dist[i]);
  }

  if(ans == 100000000){
    println(-1);
  }else println(ans/2);

  return 0;
}


Measured memory_bytes: 45164