#pragma GCC optimize ("O3")
#include "bits/stdc++.h"

using namespace std;
using ll = long long int;

#define debugos cout
#define debug(v) {printf("L%d %s > ",__LINE__,#v);debugos<<(v)<<endl;}
#define debugv(v) {printf("L%d %s > ",__LINE__,#v);for(auto e:(v)){debugos<<e<<" ";}debugos<<endl;}
#define debuga(m,w) {printf("L%d %s > ",__LINE__,#m);for(int x=0;x<(w);x++){debugos<<(m)[x]<<" ";}debugos<<endl;}
#define debugaa(m,h,w) {printf("L%d %s >\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){debugos<<(m)[y][x]<<" ";}debugos<<endl;}}
#define ALL(v) (v).begin(),(v).end()
#define repeat(cnt,l) for(remove_reference<remove_const<decltype(l)>::type>::type cnt=0;(cnt)<(l);++(cnt))
#define rrepeat(cnt,l) for(auto cnt=(l)-1;0<=(cnt);--(cnt))
#define iterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);++(cnt))
#define diterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);--(cnt))
const ll MD = 1000000007ll; const long double PI = 3.1415926535897932384626433832795L;
inline void assert_call(bool assertion, function<void()> f) { if (!assertion) { cerr << "assertion fault:" << endl; f(); abort(); } }
template<typename T1, typename T2> inline ostream& operator <<(ostream &o, const pair<T1, T2> p) { o << '(' << p.first << ':' << p.second << ')'; return o; }
template<typename Vec> inline ostream& _ostream_vecprint(ostream& os, const Vec& a) {
    os << '['; for (const auto& e : a) os << ' ' << e << ' '; os << ']'; return os;
}
template<typename T> inline ostream& operator<<(ostream& o, const vector<T>& v) { return _ostream_vecprint(o, v); }
template<typename T, size_t S> inline ostream& operator<<(ostream& o, const array<T, S>& v) { return _ostream_vecprint(o, v); }
template<typename T> inline T& maxset(T& to, const T& val) { return to = max(to, val); }
template<typename T> inline T& minset(T& to, const T& val) { return to = min(to, val); }
void bye(string s, int code = 0) { cout << s << endl; exit(code); }
mt19937_64 randdev(8901016);
template<typename T> inline T rand(T l, T h) { return uniform_int_distribution<T>(l, h)(randdev); }
template<> inline double rand<double>(double l, double h) { return uniform_real_distribution<double>(l, h)(randdev); }
template<> inline float rand<float>(float l, float h) { return uniform_real_distribution<float>(l, h)(randdev); }

#if defined(_WIN32) || defined(_WIN64)
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#elif defined(__GNUC__)
#else
#define getchar_unlocked getchar
#define putchar_unlocked putchar
#endif
namespace {
#define isvisiblechar(c) (0x21<=(c)&&(c)<=0x7E)
    class MaiScanner {
    public:
        template<typename T> void input_integer(T& var) {
            var = 0; T sign = 1;
            int cc = getchar_unlocked();
            for (; cc<'0' || '9'<cc; cc = getchar_unlocked())
                if (cc == '-') sign = -1;
            for (; '0' <= cc && cc <= '9'; cc = getchar_unlocked())
                var = (var << 3) + (var << 1) + cc - '0';
            var = var * sign;
        }
        inline int c() { return getchar_unlocked(); }
        inline MaiScanner& operator>>(int& var) { input_integer<int>(var); return *this; }
        inline MaiScanner& operator>>(long long& var) { input_integer<long long>(var); return *this; }
        inline MaiScanner& operator>>(string& var) {
            int cc = getchar_unlocked();
            for (; !isvisiblechar(cc); cc = getchar_unlocked());
            for (; isvisiblechar(cc); cc = getchar_unlocked())
                var.push_back(cc);
            return *this;
        }
        template<typename IT> void in(IT begin, IT end) { for (auto it = begin; it != end; ++it) *this >> *it; }
    };
    class MaiPrinter {
    public:
        template<typename T>
        void output_integer(T var) {
            if (var == 0) { putchar_unlocked('0'); return; }
            if (var < 0)
                putchar_unlocked('-'),
                var = -var;
            char stack[32]; int stack_p = 0;
            while (var)
                stack[stack_p++] = '0' + (var % 10),
                var /= 10;
            while (stack_p)
                putchar_unlocked(stack[--stack_p]);
        }
        inline MaiPrinter& operator<<(char c) { putchar_unlocked(c); return *this; }
        inline MaiPrinter& operator<<(int var) { output_integer<int>(var); return *this; }
        inline MaiPrinter& operator<<(long long var) { output_integer<long long>(var); return *this; }
        inline MaiPrinter& operator<<(char* str_p) { while (*str_p) putchar_unlocked(*(str_p++)); return *this; }
        inline MaiPrinter& operator<<(const string& str) {
            const char* p = str.c_str();
            const char* l = p + str.size();
            while (p < l) putchar_unlocked(*p++);
            return *this;
        }
        template<typename IT> void join(IT begin, IT end, char sep = ' ') { for (bool b = 0; begin != end; ++begin, b = 1) b ? *this << sep << *begin : *this << *begin; }
    };
}
MaiScanner scanner;
MaiPrinter printer;


// Edge構造体を定義する無向グラフ
class GraphE {
public:
    typedef int W_T;
    struct Edge {
        int u, v;
        W_T value;
        Edge(int from = 0, int to = 0, W_T value = 0) :u(from), v(to), value(value) {}
        inline int to(int _v) const { return _v == v ? u : v; }
    };
    size_t n;
    vector<vector<int>> vertex_to;
    vector<Edge> edges;

    GraphE(int n = 1) :n(n), vertex_to(n) { }

    void resize(size_t _n) {
        n = _n;
        vertex_to.resize(_n);
    }
    void connect(int from, int to, W_T val = 0) {
        vertex_to[(size_t)from].push_back((int)edges.size());
        vertex_to[(size_t)to].push_back((int)edges.size());
        edges.emplace_back(from, to, val);
    }
};

class DGraph {
public:
    size_t n;
    vector<vector<int>> vertex_to;
    vector<vector<int>> vertex_from;

    DGraph(size_t n = 1) :n(n), vertex_to(n), vertex_from(n) {}

    void connect(int from, int to) {
        vertex_to[(size_t)from].emplace_back(to);
        vertex_from[(size_t)to].emplace_back(from);
    }
    void resize(size_t _n) {
        n = _n;
        vertex_to.resize(_n);
        vertex_from.resize(_n);
    }
};


class Graph {
public:
    size_t n;
    vector<vector<int>> vertex_to;

    Graph(size_t n = 1) :n(n), vertex_to(n) {}

    void connect(int from, int to) {
        vertex_to[(size_t)from].emplace_back(to);
        vertex_to[(size_t)to].emplace_back(from);
    }
    void resize(size_t _n) {
        n = _n;
        vertex_to.resize(_n);
    }
};

class Unionfind {
public:
    vector<int> data;
    Unionfind(size_t size) : data(size, -1) { }
    bool connect(size_t x, size_t y) {
        x = root(x); y = root(y);
        if (x != y) {
            if (data[y] < data[x]) swap(x, y);
            data[x] += data[y]; data[y] = (int)x;
        }
        return x != y;
    }
    inline bool same(size_t x, size_t y) {
        return root(x) == root(y);
    }
    inline size_t root(size_t x) {
        return (size_t)(data[x] < 0 ? x : data[x] = root(data[x]));
    }
    inline int size(size_t x) {
        return -data[root(x)];
    }
};

struct Uniunifind {
    map<int, int> data;
    bool connect(int x, int y) {
        x = root(x), y = root(y);
        if (x != y) {
            data[y] = x;
        }
        return x != y;
    }
    inline bool same(int x, int y) {
        return root(x) == root(y);
    }
    int root(int x) {
        return data[x] == 0 ? x : data[x] = root(data[x]);
    }
};


int N, M;
GraphE graph;
vector<vector<int>> hyperEdge;
Graph hgraph;


vector<int> visited;



const int inf = 1e9;

int main() {

    scanner >> N >> M;
    graph.resize(N);

    map<int, vector<pair<int, int>>> rosen;

    repeat(i, M) {
        int u, v, w;
        scanner >> u >> v >> w;
        graph.connect(u - 1, v - 1, w);
        rosen[w].emplace_back(u - 1, v - 1);
    }

    visited.resize(M);

    for (auto& sen : rosen) {
        Uniunifind uf;
        for (auto p : sen.second) {
            uf.connect(p.first+1, p.second+1);
        }
        map<int, int> rt2idx;
        for (auto t : uf.data) {
            int v = t.first-1;
            int r = uf.root(t.first);
            if (rt2idx.count(r) == 0) {
                rt2idx[r] = hyperEdge.size();
                hyperEdge.emplace_back();
            }
            hyperEdge[rt2idx[r]].push_back(v);
        }
    }


    hgraph.resize(N + hyperEdge.size());
    Unionfind uni(N);

    repeat(i, int(hyperEdge.size())) {
        // debugv(hyperEdge[i]);
        for (int v : hyperEdge[i]) {
            hgraph.connect(v, N + i);
            // cerr << make_pair(v, N+i) << endl;
            uni.connect(hyperEdge[i].back(), v);
        }
    }
    if (!uni.same(0, N - 1)) bye("-1");

    visited.resize(hgraph.n);
    fill(ALL(visited), inf);
    queue<int> pque;
    pque.emplace(0);
    visited[0] = 0;

    while (!pque.empty()) {
        int v, w;
        v = pque.front(); pque.pop();
        w = visited[v];
        // cout << make_pair(w,v) << endl;
        if (v == N - 1) {
            cout << w / 2 << endl;
            return 0;
        }

        if (visited[v] < w) continue;
        for (int u : hgraph.vertex_to[v]) {
            if (visited[u] > w + 1) {
                visited[u] = w + 1;
                pque.emplace(u);
            }
        }
    }
    cout << -1 << endl;

    return 0;
}

Measured memory_bytes: 60588