#nullable enable

#region
var (_input, _iter) = (Array.Empty<string>(), 0);
T I<T>() where T : IParsable<T>
{
    while (_iter >= _input.Length) (_input, _iter) = (Console.ReadLine()!.Trim().Split(' '), 0);
    return T.Parse(_input[_iter++], null);
}
#endregion

static T[] Range<T>(int n, Func<T> F) => Enumerable.Range(0, n).Select(_ => F()).ToArray();

var n = I<int>();
var m = I<int>();
var oez = Range(m, () => (I<int>() - 1, I<int>() - 1, I<int>()));

bool F(int x)
{
    var edges = new List<(int, int)>();
    foreach (var (u, v, h) in oez) if (h >= x) edges.Add((u, v));
    var g = new StaticGraph(edges, n, false).ToGraph();
    var depth = g.Depths(0)[^1];
    return depth >= 0;
}

var pass = 0;
var fail = int.MaxValue / 2;
while (Math.Abs(pass - fail) >= 2)
{
    var mid = (pass + fail) / 2;
    if (F(mid)) pass = mid;
    else fail = mid;
}
if (pass == 0) Console.WriteLine("NaN");
else Console.WriteLine(pass);

class StaticGraph
{
    public int N { get; }
    public bool Directed { get; }
    public ReadOnlySpan<(int next, int edgeIndex)> Adjacencies(int of) => _adjacencies.AsSpan()[_starts[of].._starts[of + 1]];
    public ReadOnlySpan<(int Ab, int Ad)> Edges => _edges;

    readonly (int Ab, int Ad)[] _edges;
    readonly (int, int)[] _adjacencies;
    readonly int[] _starts;

    public StaticGraph(IReadOnlyList<(int, int)> edges, int n, bool directed)
    {
        var ez = edges.ToArray();
        var el = ez.Length;
        var m = directed ? el : (el << 1);
        var starts = new int[n + 1];
        for (var i = 0; i < el; i++)
        {
            var (ab, ad) = ez[i];
            starts[ab + 1]++;
            if (!directed) starts[ad + 1]++;
        }
        for (var i = 0; i < n; i++) starts[i + 1] += starts[i];
        var adjacencies = new (int, int)[m];
        var counts = starts.AsSpan().ToArray();
        for (var i = 0; i < el; i++)
        {
            var (ab, ad) = ez[i];
            adjacencies[counts[ab]++] = (ad, i);
            if (!directed) adjacencies[counts[ad]++] = (ab, i);
        }
        N = n;
        Directed = directed;
        _adjacencies = adjacencies;
        _edges = ez;
        _starts = starts;
    }
}

class Graph { public required StaticGraph StaticGraph { get; init; } }

static class GraphBaseExtensions
{
    public static Graph ToGraph(this StaticGraph g) => new(){ StaticGraph = g };
    public static Span<int> Depths(this Graph graph, int start)
    {
        var g = graph.StaticGraph;
        var res = new int[g.N].AsSpan();
        for (var i = 0; i < res.Length; i++) res[i] = -1;
        res[start] = 0;
        var q = new Queue<int>();
        q.Enqueue(start);
        while (q.Count > 0)
        {
            var v = q.Dequeue();
            var adjacencies = g.Adjacencies(v);
            foreach (var (next, _) in adjacencies)
            {
                if (res[next] >= 0) continue;
                res[next] = res[v] + 1;
                q.Enqueue(next);
            }
        }
        return res;
    }

}