using System.Linq;
using System.Collections.Generic;
using System;

public class P
{
    public int to { get; set; }
    public int step { get; set; }
}

public class Hello
{
    public static int n, m;
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        n = int.Parse(line[0]);
        m = int.Parse(line[1]);
        var aa = new List<int>[n];
        for (int i = 0; i < n; i++) aa[i] = new List<int>();
        for (int i = 0; i < m; i++)
        {
            line = Console.ReadLine().Trim().Split(' ');
            var a = int.Parse(line[0]) - 1;
            var b = int.Parse(line[1]) - 1;
            aa[a].Add(b);
            aa[b].Add(a);
        }
        getAns(aa);
    }
    static void getAns(List<int>[] aa)
    {
        var q = new Queue<P>();
        var visited = new bool[n];
        q.Enqueue(new P { to = 0, step = 0 });
        visited[0] = true;
        while (q.Count() > 0)
        {
            var t = q.Dequeue();
            if (t.to == n - 1) { Console.WriteLine(t.step); return; }
            foreach (var x in aa[t.to])
            {
                if (visited[x]) continue;
                q.Enqueue(new P { to = x, step = t.step + 1 });
                visited[x] = true;
            }
        }
        Console.WriteLine(-1);
    }
}