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[] NMi => ReadLine().Split().Select(c => int.Parse(c) - 1).ToArray();
    static int[][] NMap(int n) => Enumerable.Repeat(0, n).Select(_ => NMi).ToArray();
    static void Main()
    {
        var c = NList;
        var (n, q) = (c[0], c[1]);
        var map = NMap(q);

        var tree = new List<(int id, int to)>[n];
        for (var i = 0; i < tree.Length; ++i) tree[i] = new List<(int id, int to)>();
        for (var i = 0; i < map.Length; ++i)
        {
            tree[map[i][0]].Add((i, map[i][1]));
        }
        if (!HasLoop(tree, q))
        {
            WriteLine(-1);
            return;
        }
        var ok = q;
        var ng = 0;
        while (ok - ng > 1)
        {
            var center = (ng + ok) / 2;
            if (HasLoop(tree, center)) ok = center;
            else ng = center;
        }
        WriteLine(ok);
    }
    static bool HasLoop(List<(int id, int to)>[] tree, int time)
    {
        var refcount = new int[tree.Length];
        foreach (var from in tree)
        {
            foreach (var edge in from)
            {
                if (edge.id < time) ++refcount[edge.to];
            }
        }
        var q = new Queue<int>();
        for (var i = 0; i < refcount.Length; ++i) if (refcount[i] == 0) q.Enqueue(i);
        while (q.Count > 0)
        {
            var cur = q.Dequeue();
            foreach (var edge in tree[cur])
            {
                if (edge.id < time)
                {
                    --refcount[edge.to];
                    if (refcount[edge.to] == 0) q.Enqueue(edge.to);
                }
            }
        }
        return refcount.Max() > 0;
    }
}