#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
// #define int long long
// #define double long double
#define rep(i,n) for (int i=0; i<(int)(n); ++i)
#define rep1(i,n) for (int i=1; i<(int)(n); ++i)
#define repeq(i,n) for (int i=0; i<=(int)(n); ++i)
#define rep1eq(i,n) for (int i=1; i<=(int)(n); ++i)
#define rrep(i,n) for (int i=(int)(n)-1; i>=0; --i)
#define rrep1(i,n) for (int i=(int)(n)-1; i>0; --i)
#define rrepeq(i,n) for (int i=(int)(n); i>=0; --i)
#define rrep1eq(i,n) for (int i=(int)(n); i>0; --i)
#define REP(i,a,b) for (int i=(int)(a); i<=(int)(b); ++i)
#define RREP(i,a,b) for (int i=(int)(a); i>=(int)(b); --i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vb = vector<bool>;
template<typename T> using Graph = vector<vector<T>>;
template<typename T> using Spacial = vector<vector<vector<T>>>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int MOD = 1e9+7;
const int MOD2 = 998244353;
// const double EPS = 1e-9;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
string interval[2] = {" ", "\n"}; // {" ", "\n"}

template<typename T> struct is_plural : false_type{};
template<typename T1, typename T2> struct is_plural<pair<T1, T2>> : true_type{};
template<typename T> struct is_plural<vector<T>> : true_type{};
template<typename T> struct is_plural<complex<T>> : true_type{};
 
template<typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p) { return is >> p.first >> p.second; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p) { return os << p.first << " " << p.second; }
template<typename T> istream &operator>>(istream &is, vector<T> &vec) { for (auto itr = vec.begin(); itr != vec.end(); ++itr) is >> *itr; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &vec) { if (vec.empty()) return os; bool pl = is_plural<T>(); os << vec.front(); for (auto itr = ++vec.begin(); itr != vec.end(); ++itr) os << interval[pl] << *itr; return os; }
 
bool CoutYN(bool a, string y = "Yes", string n = "No") { cout << (a ? y : n) << "\n"; return a; }

template<typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template<typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

long long modpow(int a, long long n, int mod = MOD) { long long ret = 1; do { if (n & 1) ret = ret * a % mod; a = 1LL * a * a % mod; } while (n >>= 1); return ret; }

template<typename T> T GCD(T a, T b) { return b ? GCD(b, a%b) : a; }
template<typename T> T LCM(T a, T b) { return a / GCD(a, b) * b; }

template<typename T1, typename T2> bool CompareBySecond(pair<T1, T2> a, pair<T1, T2> b) { return a.second != b.second ? a.second < b.second : a.first < b.first; }
template<typename T1, typename T2> bool CompareByInverse(pair<T1, T2> a, pair<T1, T2> b) { return a.first != b.first ? a.first < b.first : a.second > b.second; }


/* -------- <templates end> -------- */


template<typename T>
struct edge {
  int from, to;
  T cost;

  edge(int to, T cost) : from(-1), to(to), cost(cost) {}

  edge(int from, int to, T cost) : from(from), to(to), cost(cost) {}

  bool operator<(const edge<T> &e) const {
    return cost < e.cost;
  }

  bool operator<(const int &n) const {
    return cost < n;
  }
};

template<typename T> using Edges = vector<edge<T>>;
template<typename T> using WeightedGraph = vector<vector<edge<T>>>;

const int INFTY = -1e6;

void solve() {
  int n, m; cin >> n >> m;
  WeightedGraph<int> g(n);
  rep(i,m) {
    int p, q, c; cin >> p >> q >> c; --p, --q;
    g[p].emplace_back(q, c);
    g[q].emplace_back(p, c);
  }

  for (auto &vec : g) sort(ALL(vec));

  priority_queue<edge<int>> pq;
  vi dist(n, INFTY);
  vector<set<int>> train(n);

  pq.emplace(0, 0);
  dist[0] = 0;

  while (!pq.empty()) {
    auto e = pq.top(); pq.pop();
    if (dist[e.to] != e.cost) continue;

    for (auto &ne : g[e.to]) {
      if (chmax(dist[ne.to], e.cost - 1)) {
        pq.emplace(ne.to, dist[ne.to]);
        train[ne.to].emplace(ne.cost);
      } else if (dist[ne.to] > e.cost - 1 || !train[ne.to].emplace(ne.cost).second) {
        continue;
      }
      auto dfs = [&](auto dfs, int v) -> void {
        auto itr = lower_bound(ALL(g[v]), ne.cost);
        while (itr != g[v].end() && itr->cost == ne.cost) {
          if (dist[itr->to] == dist[ne.to] && train[itr->to].emplace(itr->cost).second) {
            dfs(dfs, itr->to);
          } else if (chmax(dist[itr->to], dist[ne.to])) {
            pq.emplace(itr->to, dist[ne.to]);
            train[itr->to].emplace(itr->cost).second;
            dfs(dfs, itr->to);
          }
          ++itr;
        }
      };
      dfs(dfs, ne.to);
    }
  }

  if (dist[n-1] != INFTY) cout << -dist[n-1] << endl;
  else cout << -1 << endl;
}


/* -------- <programs end> -------- */


signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  solve();
  return 0;
}


Measured memory_bytes: 25012