#include <bits/stdc++.h>
#define EPS 1e-9
#define INF 1070000000LL
#define MOD 1000000007LL
#define fir first
#define foreach(it,X) for(auto it=(X).begin();it!=(X).end();it++)
#define numa(x,a) for(auto x: a)
#define ite iterator
#define mp make_pair
#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<(n);i++)
#define pb push_back
#define pf push_front
#define sec second
#define sz(x) ((int)(x).size())
#define ALL( c ) (c).begin(), (c).end()
#define gcd(a,b) __gcd(a,b)
#define mem(x,n) memset(x,n,sizeof(x))
#define endl "\n"
using namespace std;
template <int POS, class TUPLE> void deploy(std::ostream &os, const TUPLE &tuple){}
template <int POS, class TUPLE, class H, class ...Ts> void deploy(std::ostream &os, const TUPLE &t){ os << (POS == 0 ? "" : ", ") << get<POS>(t); deploy<POS + 1, TUPLE, Ts...>(os, t); }
template <class T,class U> std::ostream& operator<<(std::ostream &os, std::pair<T,U> &p){ os << "(" << p.first <<", " << p.second <<")";return os; }
template <class T> std::ostream& operator<<(std::ostream &os, std::vector<T> &v){ int remain = v.size(); os << "{"; for(auto e: v) os << e << (--remain == 0 ? "}" : ", "); return os; }
template <class T> std::ostream& operator<<(std::ostream &os, std::set<T> &v){ int remain = v.size(); os << "{"; for(auto e: v) os << e << (--remain == 0 ? "}" : ", "); return os; }
template <class T, class K> std::ostream& operator<<(std::ostream &os, std::map<T, K> &mp){ int remain = mp.size(); os << "{"; for(auto e: mp) os << "(" << e.first << " -> " << e.second << ")" << (--remain == 0 ? "}" : ", "); return os; }
#define DEBUG1(var0) { std::cerr << (#var0) << "=" << (var0) << endl; }
#define DEBUG2(var0, var1) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG1(var1); }
#define DEBUG3(var0, var1, var2) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG2(var1,var2); }
#define DEBUG4(var0, var1, var2, var3) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG3(var1,var2,var3); }
using ll = long long;

vector <vector <pair <pair <int,int>,int> > > edges[100100];
vector <pair <int,int> > inputs[1000100];
vector <int> cost[100100];
int main()
{
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  int N,M;
  cin >> N >> M;

  rep(i,M){
    int x,y,c;
    cin >> x >> y >> c;
    x--;y--;
    inputs[c].pb(mp(x,y));
  }
  vector <pair <pair <int,int>,int> > empv;
  rep(i,1000010){
    if (sz(inputs[i]) > 0) {
      set <int> stat;
      numa(x,inputs[i]){
        stat.insert(x.fir);
        stat.insert(x.sec);
      }
      numa(s, stat){
        edges[s].push_back(empv);
      }
      numa(e,inputs[i]){
        int x = e.fir, y = e.sec;
        int szx = sz(edges[x]);
        int szy = sz(edges[y]);
        edges[x][szx-1].push_back(mp(mp(y,szy-1), 0));
        edges[y][szy-1].push_back(mp(mp(x,szx-1),0));
      }
    }
  }
  rep(j,100010){
    if (sz(edges[j]) > 0) {
      int n = sz(edges[j]);
      edges[j].push_back(empv);
      rep(i,n){
        edges[j][i].push_back(mp(mp(j,n),1));
        edges[j][n].push_back(mp(mp(j,i),1));
      }
      cost[j].resize(n+1, -1);
    }
  }
  if (sz(edges[0]) == 0 || sz(edges[N-1]) == 0) {
    cout << -1 << endl;
    return 0;
  }
  assert(sz(edges[0]) >= 2);
  assert(sz(edges[N-1]) >= 2);
  pair <int,int> start = mp(0, sz(edges[0]) - 1);
  pair <int,int> goal = mp(N-1, sz(edges[N-1]) - 1);
  using T = pair <int, pair <int,int> >;
  priority_queue <T, vector <T>, greater <T> > que;
  que.push(mp(0,start));
  int ans = -1;
  while(!que.empty()){
    auto t = que.top();
    que.pop();
    if (t.sec == goal){
      ans = t.fir;
      break;
    }
    int c = t.fir;
    int x = t.sec.fir, y = t.sec.sec;
    //DEBUG3(x,y,c);
    if (cost[x][y] != -1) {
      continue;
    }
    cost[x][y] = c;
    numa(z,edges[x][y]){
      int nx = z.fir.fir;
      int ny = z.fir.sec;
      int pc = z.sec;
      if (cost[nx][ny] == -1) {
        que.push(mp(c + pc, mp(nx,ny)));
      }
    }
  }
  if (ans == -1) {
    cout << -1 << endl;
  }else{
    cout << ans / 2 << endl;
  }
  return 0;
}



Measured memory_bytes: 75768