using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using SB = System.Text.StringBuilder;
//using System.Threading.Tasks;
//using System.Text.RegularExpressions;
//using System.Globalization;
//using System.Diagnostics;
using static System.Console;
using System.Numerics;
using static System.Math;
using pair = Pair<int, int>;

class Program
{
    static void Main()
    {
        //SetOut(new StreamWriter(OpenStandardOutput()) { AutoFlush = false });
        new Program().solve();
        Out.Flush();
    }
    readonly Scanner cin = new Scanner();
    readonly int[] dd = { 0, 1, 0, -1, 0 }; //→↓←↑
    readonly int mod = 1000000007;
    readonly int dom = 998244353;
    bool chmax<T>(ref T a, T b) where T : IComparable<T> { if (a.CompareTo(b) < 0) { a = b; return true; } return false; }
    bool chmin<T>(ref T a, T b) where T : IComparable<T> { if (b.CompareTo(a) < 0) { a = b; return true; } return false; }

    void solve()
    {
        int N = cin.nextint;
        int M = cin.nextint;
        var D = new Dijkstra();
        var G = D.Input(N, M, true, 1, false);
        int S = cin.nextint - 1;
        int T = cin.nextint - 1;
        var dist = D.Run(G, S);
        if (dist[T, 0] == D.Inf)
        {
            WriteLine(-1);
        }
        else
        {
            WriteLine(dist[T, 0] / 3);
        }

    }

}


class Dijkstra
{
    public readonly long Inf;
    public Dijkstra(long Inf = long.MaxValue / 2) { this.Inf = Inf; }

    /// <summary>
    /// VertexとEdgeを兼ねる
    /// </summary>
    public struct Edge : IComparable<Edge>
    {
        public int to; public long cost;
        public Edge(int to, long cost)
        {
            this.to = to;
            this.cost = cost;
        }
        public int CompareTo(Edge e) => cost.CompareTo(e.cost);
    }
    /// <summary>
    /// Dijkstraな入力を行う
    /// </summary>
    /// <param name="n">頂点数</param>
    /// <param name="m">辺数</param>
    /// <param name="dir">有向/無向 デフォルト: 無向</param>
    /// <param name="indexed">indexedの補正 デフォルト: 1</param>
    /// <returns></returns>
    public List<Edge>[] Input(int n, int m, bool dir = false, int indexed = 1, bool cost = true)
    {
        Scanner cin = new Scanner();
        var ret = new List<Edge>[n];
        for (int i = 0; i < n; i++)
        {
            ret[i] = new List<Edge>();
        }
        for (int i = 0; i < m; i++)
        {
            int a = cin.nextint - indexed;
            int b = cin.nextint - indexed;
            var c = cost ? cin.nextlong : 1;
            ret[a].Add(new Edge(b, c));
            if (!dir) ret[b].Add(new Edge(a, c));
        }
        return ret;
    }
    public long[,] Run(List<Edge>[] edges, int s)
    {
        int n = edges.Length;
        var dist = new long[n, 3];
        var Q = new PriorityQueue<Edge>(-1);
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < 3; j++)
            {
                dist[i, j] = Inf;
            }
        }
        dist[s, 0] = 0;
        Q.Enqueue(new Edge(s, 0));
        while (Q.Any)
        {
            var p = Q.Dequeue();
            var v = p.to;
            int z = (int)(p.cost % 3);
            if (dist[v, z] < p.cost) continue;
            foreach (var e in edges[v])
            {
                var d = p.cost + e.cost;
                int f = (int)(d % 3);
                if (d < dist[e.to, f])
                {
                    dist[e.to, f] = d;
                    Q.Enqueue(new Edge(e.to, d));
                }
            }
        }
        return dist;
    }


    /// <summary>
    /// 最短路 経路総数 Modとり
    /// </summary>
    /// <param name="edges"></param>
    /// <param name="s"></param>
    /// <param name="cnt"></param>
    /// <param name="Mod"></param>
    /// <returns></returns>
    public long[] Run(List<Edge>[] edges, int s, out long[] cnt, long Mod)
    {
        int n = edges.Length;
        var dist = new long[n];
        var Q = new PriorityQueue<Edge>(-1);
        for (int i = 0; i < n; i++) dist[i] = Inf;
        dist[s] = 0;
        Q.Enqueue(new Edge(s, dist[s]));
        cnt = new long[n];
        cnt[s] = 1;
        while (Q.Any)
        {
            var p = Q.Dequeue();
            var v = p.to;
            if (dist[v] < p.cost) continue;
            foreach (var e in edges[v])
            {
                var d = p.cost + e.cost;
                if (d < dist[e.to])
                {
                    dist[e.to] = d;
                    cnt[e.to] = cnt[p.to];
                    Q.Enqueue(new Edge(e.to, d));
                }
                else if (d == dist[e.to])
                {
                    cnt[e.to] = (cnt[e.to] + cnt[p.to]) % Mod;
                }
            }
        }
        return dist;
    }
}
/// <summary>
/// Original Author: 蟻本
/// </summary>
/// <typeparam name="T"></typeparam>
class PriorityQueue<T> where T : IComparable<T>
{
    List<T> heap;
    readonly int cmp;
    public PriorityQueue(int cmp = 1) { heap = new List<T>(); this.cmp = cmp; }
    public void Enqueue(T x)
    {
        heap.Add(x);
        int i = Count++;
        while (i > 0)
        {
            int p = (i - 1) >> 1;
            if (x.CompareTo(heap[p]) * cmp <= 0) break;
            heap[i] = heap[p]; i = p;
        }
        heap[i] = x;
    }
    public T Dequeue()
    {
        T ret = heap[0]; T x = heap[--Count];
        int i = 0;
        while ((i << 1) + 1 < Count)
        {
            int a = (i << 1) + 1; int b = (i << 1) + 2;
            if (b < Count && heap[a].CompareTo(heap[b]) * cmp < 0) a = b;
            if (x.CompareTo(heap[a]) * cmp >= 0) break;
            heap[i] = heap[a]; i = a;
        }
        heap[i] = x; heap.RemoveAt(Count);
        return ret;
    }
    public int Count { get; private set; }
    public bool Any => Count > 0;
    public T Peek() => heap[0];
}

static class Ex
{
    public static void join<T>(this IEnumerable<T> values, string sep = " ") => WriteLine(string.Join(sep, values));
    public static string concat<T>(this IEnumerable<T> values) => string.Concat(values);
    public static string reverse(this string s) { var t = s.ToCharArray(); Array.Reverse(t); return t.concat(); }

    public static int lower_bound<T>(this IList<T> arr, T val) where T : IComparable<T>
    {
        int low = 0, high = arr.Count;
        int mid;
        while (low < high)
        {
            mid = ((high - low) >> 1) + low;
            if (arr[mid].CompareTo(val) < 0) low = mid + 1;
            else high = mid;
        }
        return low;
    }
    public static int upper_bound<T>(this IList<T> arr, T val) where T : IComparable<T>
    {
        int low = 0, high = arr.Count;
        int mid;
        while (low < high)
        {
            mid = ((high - low) >> 1) + low;
            if (arr[mid].CompareTo(val) <= 0) low = mid + 1;
            else high = mid;
        }
        return low;
    }
}

class Pair<T, U> : IComparable<Pair<T, U>> where T : IComparable<T> where U : IComparable<U>
{
    public T f; public U s;
    public Pair(T f, U s) { this.f = f; this.s = s; }
    public int CompareTo(Pair<T, U> a) => f.CompareTo(a.f) != 0 ? f.CompareTo(a.f) : s.CompareTo(a.s);
    public override string ToString() => $"{f} {s}";
}

class Scanner
{
    string[] s; int i;
    readonly char[] cs = new char[] { ' ' };
    public Scanner() { s = new string[0]; i = 0; }
    public string[] scan => ReadLine().Split();
    public int[] scanint => Array.ConvertAll(scan, int.Parse);
    public long[] scanlong => Array.ConvertAll(scan, long.Parse);
    public double[] scandouble => Array.ConvertAll(scan, double.Parse);
    public string next
    {
        get
        {
            if (i < s.Length) return s[i++];
            string st = ReadLine();
            while (st == "") st = ReadLine();
            s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            i = 0;
            return next;
        }
    }
    public int nextint => int.Parse(next);
    public long nextlong => long.Parse(next);
    public double nextdouble => double.Parse(next);
}


Measured memory_bytes: 35976