using System;
using System.Collections.Generic;
using System.IO;

namespace Sandbox
{
    internal class Program
    {
        public struct Pair
        {
            public int Index;
            public int Distance;
        }

        private static void Main(string[] args)
        {
            var scanner = new Scanner();
            var n = scanner.GetNextAsInt();
            var m = scanner.GetNextAsInt();

            var u2v = new Dictionary<int, SortedSet<int>>();
            for (var i = 0; i < m; i++)
            {
                var ui = scanner.GetNextAsInt() - 1;
                var vi = scanner.GetNextAsInt() - 1;

                if (!u2v.ContainsKey(ui))
                {
                    u2v[ui] = new SortedSet<int>();
                }

                u2v[ui].Add(vi);
            }

            var s = scanner.GetNextAsInt() - 1;
            var t = scanner.GetNextAsInt() - 1;

            var dist = new int[n, 3];

            var queue = new Queue<Pair>();
            queue.Enqueue(new Pair(){Index = s, Distance = 0});
            var visited = false;

            while (queue.Count > 0)
            {
                var current = queue.Dequeue();

                if (!u2v.ContainsKey(current.Index))
                {
                    continue;
                }

                foreach (var vi in u2v[current.Index])
                {
                    var distance = current.Distance + 1;
                    var mod = distance % 3;
                    if (dist[vi, mod] != 0)
                    {
                        continue;
                    }

                    dist[vi, mod] = distance;

                    if (vi == t && mod == 0)
                    {
                        visited = true;
                        break;
                    }

                    queue.Enqueue(new Pair(){Index = vi, Distance = distance});
                }

                if (visited)
                {
                    break;
                }
            }

            var result = dist[t, 0] / 3;
            if (result == 0)
            {
                result = -1;
            }

            Console.WriteLine(result);
        }

        #region String Scanner

        public class Scanner
        {
            private const char Separator = ' ';
            private readonly StreamReader _Stream = new StreamReader(Console.OpenStandardInput());
            private int _Pos;
            private string[] _Line = new string[0];

            #region get a element

            public string GetNext()
            {
                // ReSharper disable once InvertIf
                if (this._Pos >= this._Line.Length)
                {
                    // ReSharper disable once PossibleNullReferenceException
                    this._Line = this._Stream.ReadLine().Split(Separator);
                    this._Pos = 0;
                }

                return this._Line[this._Pos++];
            }

            public int GetNextAsInt()
            {
                return int.Parse(this.GetNext());
            }

            public long GetNextAsLong()
            {
                return long.Parse(this.GetNext());
            }

            public double GetNextAsDouble()
            {
                return double.Parse(this.GetNext());
            }

            #endregion

            #region get array

            public string[] ToArrayFromNextLine()
            {
                if (this._Pos >= this._Line.Length)
                {
                    // ReSharper disable once PossibleNullReferenceException
                    this._Line = this._Stream.ReadLine().Split(Separator);
                }

                this._Pos = this._Line.Length;
                return this._Line;
            }

            public int[] ToIntArrayFromNextLine()
            {
                return ToIntArray(this.ToArrayFromNextLine());
            }

            public long[] ToLongArrayFromNextLine()
            {
                return ToLongArray(this.ToArrayFromNextLine());
            }

            public double[] ToDoubleArrayFromNextLine()
            {
                return ToDoubleArray(this.ToArrayFromNextLine());
            }

            #endregion

            #region convert array

            private static int[] ToIntArray(string[] array)
            {
                var result = new int[array.Length];
                for (var i = 0; i < array.Length; i++)
                {
                    result[i] = int.Parse(array[i]);
                }

                return result;
            }

            private static long[] ToLongArray(string[] array)
            {
                var result = new long[array.Length];
                for (var i = 0; i < array.Length; i++)
                {
                    result[i] = long.Parse(array[i]);
                }

                return result;
            }

            private static double[] ToDoubleArray(string[] array)
            {
                var result = new double[array.Length];
                for (var i = 0; i < array.Length; i++)
                {
                    result[i] = double.Parse(array[i]);
                }

                return result;
            }

            #endregion
        }

        #endregion
    }
}


Measured memory_bytes: 37432