using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    static int[] NMi => ReadLine().Split().Select(c => int.Parse(c) - 1).ToArray();
    static int[][] NMap(int n) => Enumerable.Repeat(0, n).Select(_ => NMi).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (n, m) = (c[0], c[1]);
        var map = NMap(m);
        var tree = new List<int>[n];
        for (var i = 0; i < tree.Length; ++i) tree[i] = new List<int>();
        foreach (var edge in map) tree[edge[0]].Add(edge[1]);

        var len1 = Len(0, tree);
        var lenn1 = Len(n - 2, tree);
        var lenn = Len(n - 1, tree);
        var ans = Math.Min(len1[n - 2] + lenn1[n - 1] + lenn[0], len1[n - 1] + lenn[n - 2] + lenn1[0]);
        WriteLine(ans >= int.MaxValue / 10 ? -1 : ans);
    }
    static int[] Len(int s, List<int>[] tree)
    {
        var ans = Enumerable.Repeat(int.MaxValue / 10, tree.Length).ToArray();
        ans[s] = 0;
        var q = new Queue<int>();
        q.Enqueue(s);
        while (q.Count > 0)
        {
            var cur = q.Dequeue();
            foreach (var next in tree[cur])
            {
                if (ans[next] <= ans[cur] + 1) continue;
                ans[next] = ans[cur] + 1;
                q.Enqueue(next);
            }
        }
        return ans;
    }
}