// {{{ by shik
#include <bits/stdc++.h>
#include <unistd.h>
#define SZ(x) ((int)(x).size())
#define ALL(x) begin(x),end(x)
#define REP(i,n) for ( int i=0; i<int(n); i++ )
#define REP1(i,a,b) for ( int i=(a); i<=int(b); i++ )
#define FOR(it,c) for ( auto it=(c).begin(); it!=(c).end(); it++ )
#define MP make_pair
#define PB push_back
using namespace std;
typedef long long LL;
typedef pair<int,int> PII;
typedef vector<int> VI;

#ifdef SHIK
template<typename T>
void _dump( const char* s, T&& head ) { cerr<<s<<"="<<head<<endl; }

template<typename T, typename... Args>
void _dump( const char* s, T&& head, Args&&... tail ) {
    int c=0;
    while ( *s!=',' || c!=0 ) {
        if ( *s=='(' || *s=='[' || *s=='{' ) c++;
        if ( *s==')' || *s==']' || *s=='}' ) c--;
        cerr<<*s++;
    }
    cerr<<"="<<head<<", ";
    _dump(s+1,tail...);
}

#define dump(...) do { \
    fprintf(stderr, "%s:%d - ", __PRETTY_FUNCTION__, __LINE__); \
    _dump(#__VA_ARGS__, __VA_ARGS__); \
} while (0)

template<typename Iter>
ostream& _out( ostream &s, Iter b, Iter e ) {
    s<<"[";
    for ( auto it=b; it!=e; it++ ) s<<(it==b?"":" ")<<*it;
    s<<"]";
    return s;
}

template<typename A, typename B>
ostream& operator <<( ostream &s, const pair<A,B> &p ) { return s<<"("<<p.first<<","<<p.second<<")"; }
template<typename T>
ostream& operator <<( ostream &s, const vector<T> &c ) { return _out(s,ALL(c)); }
template<typename T, size_t N>
ostream& operator <<( ostream &s, const array<T,N> &c ) { return _out(s,ALL(c)); }
template<typename T>
ostream& operator <<( ostream &s, const set<T> &c ) { return _out(s,ALL(c)); }
template<typename A, typename B>
ostream& operator <<( ostream &s, const map<A,B> &c ) { return _out(s,ALL(c)); }
#else
#define dump(...)
#endif

template<typename T>
void _R( T &x ) { cin>>x; }
void _R( int &x ) { scanf("%d",&x); }
void _R( long long &x ) { scanf("%" PRId64,&x); }
void _R( double &x ) { scanf("%lf",&x); }
void _R( char &x ) { scanf(" %c",&x); }
void _R( char *x ) { scanf("%s",x); }

void R() {}
template<typename T, typename... U>
void R( T& head, U&... tail ) {
    _R(head);
    R(tail...);
}

template<typename T>
void _W( const T &x ) { cout<<x; }
void _W( const int &x ) { printf("%d",x); }
template<typename T>
void _W( const vector<T> &x ) {
    for ( auto i=x.cbegin(); i!=x.cend(); i++ ) {
        if ( i!=x.cbegin() ) putchar(' ');
        _W(*i);
    }
}

void W() {}
template<typename T, typename... U>
void W( const T& head, const U&... tail ) {
    _W(head);
    putchar(sizeof...(tail)?' ':'\n');
    W(tail...);
}

#ifdef SHIK
#define FILEIO(...)
#else
#define FILEIO(name) do {\
    freopen(name ".in","r",stdin); \
    freopen(name ".out","w",stdout); \
} while (0)
#endif

// }}}

const int N=2e6+10;
const int M=2e5+10;

struct E {
    int a,b,c;
} es[M];

int n,m,nn;
bool vis[N];
VI ec[N],e[N],ee[N];

vector<int> q;
int dis[N];
void BFS( int st ) {
    q.clear();
    dis[st]=0;
    vis[st]=1;
    q.PB(st);
    for ( int i=0; i<SZ(q); i++ ) {
        int p=q[i];
        for ( int j:e[p] ) if ( !vis[j] ) {
            vis[j]=1;
            dis[j]=dis[p]+1;
            q.PB(j);
        }
    }
}

int main() {
    R(n,m);
    REP(i,m) R(es[i].a,es[i].b,es[i].c);
    REP(i,m) ec[es[i].c].PB(i);
    nn=n;
    REP(i,N) {
        VI vc;
        for ( int j:ec[i] ) {
            vc.PB(es[j].a);
            vc.PB(es[j].b);
        }
        sort(ALL(vc));
        vc.erase(unique(ALL(vc)),vc.end());
        for ( int j:vc ) {
            vis[j]=0;
            e[j].clear();
        }
        for ( int j:ec[i] ) {
            e[es[j].a].PB(es[j].b);
            e[es[j].b].PB(es[j].a);
        }
        for ( int j:vc ) if ( !vis[j] ) {
            BFS(j);
            nn++;
            for ( int k:q ) {
                ee[k].PB(nn);
                ee[nn].PB(k);
            }
        }
    }
    memset(vis,0,sizeof(vis));
    REP1(i,1,nn) e[i]=ee[i];
    BFS(1);
    if ( !vis[n] ) W(-1);
    else {
        int ans=dis[n]/2;
        W(ans);
    }
    return 0;
}


Measured memory_bytes: 173688