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

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) = (c[0], c[1]);
        var map = NArr(m);
        var tree = new List<(int to, int h)>[n];
        for (var i = 0; i < tree.Length; ++i) tree[i] = new List<(int to, int h)>();
        foreach (var edge in map)
        {
            tree[edge[0] - 1].Add((edge[1] - 1, edge[2]));
            tree[edge[1] - 1].Add((edge[0] - 1, edge[2]));
        }
        var ok = 0;
        var ng = 1_000_000_001;
        while (ng - ok > 1)
        {
            var mid = (ok + ng) / 2;
            var q = new Queue<int>();
            q.Enqueue(0);
            var visited = new bool[n];
            visited[0] = true;
            while (q.Count > 0)
            {
                var cur = q.Dequeue();
                foreach (var next in tree[cur])
                {
                    if (mid > next.h) continue;
                    if (visited[next.to]) continue;
                    visited[next.to] = true;
                    q.Enqueue(next.to);
                }
            }
            if (visited[^1]) ok = mid;
            else ng = mid;
        }
        WriteLine(ok == 0 ? "NaN" : ok.ToString());
    }
}