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();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (n, m, a, b) = (c[0], c[1], c[2], c[3]);
        var map = NArr(m);

        var tree = new List<int>[n + 2];
        for (var i = 0; i < tree.Length; ++i) tree[i] = new List<int>();
        foreach (var edge in map)
        {
            tree[edge[0]].Add(edge[1] + 1);
            tree[edge[1] + 1].Add(edge[0]);
        }
        var INF = int.MaxValue / 2;
        var len = Enumerable.Repeat(INF, n + 2).ToArray();
        var q = new Queue<(int pos, int len)>();
        for (var i = 0; i <= a; ++i)
        {
            len[i] = 0;
            q.Enqueue((i, 0));
        }
        while (q.Count > 0)
        {
            var cur = q.Dequeue();
            foreach (var next in tree[cur.pos])
            {
                if (len[next] <= len[cur.pos] + 1) continue;
                len[next] = len[cur.pos] + 1;
                q.Enqueue((next, len[next]));
            }
        }
        var ans = len.Skip(b + 1).Min();
        WriteLine(ans == INF ? -1 : ans);
    }
}