/*

                   _ooOoo_
                  o8888888o
                  88" . "88
                  (| -_- |)
                  O\  =  /O
               ____/`---'\____
             .'  \\|     |//  `.
            /  \\|||  :  |||//  \
           /  _||||| -:- |||||-  \
           |   | \\\  -  /// |   |
           | \_|  ''\---/''  |   |
           \  .-\__  `-`  ___/-. /
         ___`. .'  /--.--\  `. . __
      ."" '<  `.___\_<|>_/___.'  >'"".
     | | :  `- \`.;`\ _ /`;.`/ - ` : | |
     \  \ `-.   \_ __\ /__ _/   .-` /  /
======`-.____`-.___\_____/___.-`____.-'======
                   `=---='
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                    prayer
*/

// g++ -std=c++11 a.cpp
#include<iostream>
#include<vector>
#include<string>
#include<algorithm>	
#include<map>
#include<set>
#include<unordered_map>
#include<utility>
#include<cmath>
#include<random>
#include<cstring>
#include<queue>
#include<stack>
#include<bitset>
#include<cstdio>
#include<sstream>
#include<iomanip>
#include<assert.h>
#include<typeinfo>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define FOR(i,a) for(auto i:a)
#define pb push_back
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)
#define show1d(v) rep(_,v.size())cout<<" "<<v[_];cout<<endl;
#define show2d(v) rep(__,v.size()){rep(_,v[__].size())cout<<" "<<v[__][_];cout<<endl;}
using namespace std;
//kaewasuretyuui
typedef long long ll;
#define int ll
typedef int Def;
typedef pair<Def,Def> pii;
typedef vector<Def> vi;
typedef vector<vi> vvi;
typedef vector<pii> vp;
typedef vector<vp> vvp;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef pair<Def,pii> pip;
typedef vector<pip>vip;
// #define mt make_tuple
// typedef tuple<int,int,int> tp;
// typedef vector<tp> vt;
template<typename A,typename B>bool cmin(A &a,const B &b){return a>b?(a=b,true):false;}
template<typename A,typename B>bool cmax(A &a,const B &b){return a<b?(a=b,true):false;}
//template<class C>constexpr int size(const C &c){return (int)c.size();}
//template<class T,size_t N> constexpr int size(const T (&xs)[N])noexcept{return (int)N;}
const double PI=acos(-1);
const double EPS=1e-9;
Def inf = sizeof(Def) == sizeof(long long) ? 2e18 : 1e9+10;
int dx[]={0,1,0,-1};
int dy[]={1,0,-1,0};
class DIJ{
	public:
	struct edge{
		int to,cost;
	};
	vector<vector<edge> >G;
	int n;
	vi d;//distance
	DIJ(int size){
		n=size+1000010;
		G=vector<vector<edge> >(n);
	}
	void add_edge(int a,int b,int c){
		edge e={b,c},ee={a,c};
		G[a].pb(e);
		G[b].pb(ee);
	}
	void dij(int s){
		d=vi(n,inf);
		d[s]=0;
		priority_queue<pii>q;
		q.push(pii(0,s));
		while(!q.empty()){
			pii p=q.top();
			q.pop();
			int pos=p.second,cost=-p.first;
			if(cost>d[pos])continue;
			rep(i,G[pos].size()){
				edge e=G[pos][i];
				int to=e.to;
				int nowcost=cost+e.cost;
				if(nowcost<d[to]){
					d[to]=nowcost;
					q.push(pii(-d[to],to));
				}
			}
		}
	}
};
int w;
struct UnionFind{
    vector<int> data;
	vi t,no;
    UnionFind(int size) : data(size, -1),no(size) {}
    bool unionSet(int x,int y){
		t.pb(x);
		t.pb(y);
        x=root(x);y=root(y);
        if(x==y)return 0;
        if(data[y]<data[x])swap(x,y);
        data[x]+=data[y],data[y]=x;
        return 1;
    }
    bool findSet(int x,int y){
        return root(x)==root(y);
    }
    int root(int x){
        return data[x]<0 ? x : data[x]=root(data[x]);
    }
    int size(int x){
        return -data[root(x)];
    }
	void uniquee(){
		t.erase(unique(all(t)),t.end());
		rep(i,t.size())if(data[t[i]]<0)no[t[i]]=w++;
	}
	void reset(){
		rep(i,t.size()){
			data[t[i]]=-1;
			no[t[i]]=0;
		}
		t=vi(0);
	}
};
int n,m;
vvp c(1000010);
signed main(){
	int n,m;
	cin>>n>>m;
	w=n;
	DIJ dij(n);
	while(m--){
		int a,b,d;
		cin>>a>>b>>d;
		a--;b--;d--;
		c[d].pb({a,b});
	}
	UnionFind uf(n);
	rep(i,1000010){
		rep(j,c[i].size())
			uf.unionSet(c[i][j].first,c[i][j].second);
		uf.uniquee();
		rep(j,uf.t.size())dij.add_edge(uf.t[j],uf.no[uf.root(uf.t[j])],1);
		uf.reset();
	}
	
	dij.dij(0);
	if(dij.d[n-1]==inf)cout<<-1<<endl;
	else cout<<dij.d[n-1]/2<<endl;
}














Measured memory_bytes: 87280