Problem:
Score : 400 points

Problem Statement

There is a directed graph with N vertices and M edges.
The i-th edge (1≤i≤M) points from vertex a_i to vertex b_i, and has a weight c_i.
We will play the following single-player game using this graph and a piece.

Initially, the piece is placed at vertex 1, and the score of the player is set to 0.
The player can move the piece as follows:

When the piece is placed at vertex a_i, move the piece along the i-th edge to vertex b_i. After this move, the score of the player is increased by c_i.

The player can end the game only when the piece is placed at vertex N.
The given graph guarantees that it is possible to traverse from vertex 1 to vertex N.

When the player acts optimally to maximize the score at the end of the game, what will the score be?
If it is possible to increase the score indefinitely, print inf.

Constraints

2≤N≤1000

1≤M≤min(N(N-1),2000)

1≤a_i,b_i≤N (1≤i≤M)

a_i≠b_i (1≤i≤M)

a_i≠a_j or b_i≠b_j (1≤i<j≤M)

-10^9≤c_i≤10^9 (1≤i≤M)

c_i is an integer.

In the given graph, there exists a path from vertex 1 to vertex N.

Input

Input is given from Standard Input in the following format:

N M
a_1 b_1 c_1
a_2 b_2 c_2
:
a_M b_M c_M

Output

Print the maximum possible score at the end of the game, if it is finite. If it is possible to increase the score indefinitely, print inf.

Sample Input 1

3 3
1 2 4
2 3 3
1 3 5

Sample Output 1

7

There are two ways to move the piece to vertex N=3:

vertex 1 → vertex 2 → vertex 3 : score 4+3=7

vertex 1 → vertex 3 : score 5

Thus, the maximum possible score at the end of the game is 7.

Sample Input 2

2 2
1 2 1
2 1 1

Sample Output 2

inf

It is possible to increase the score indefinitely by alternating between vertex 1 and 2.

Sample Input 3

6 5
1 2 -1000000000
2 3 -1000000000
3 4 -1000000000
4 5 -1000000000
5 6 -1000000000

Sample Output 3

-5000000000

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

#pragma warning disable

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.Serialization.Formatters.Binary;
using System.Text;
using System.Text.RegularExpressions;
using System.Diagnostics;
using System.Numerics;
using System.Collections;
using System.Timers;

static class MainClass
{
    public static void Main(string[] args)
    {
        #region
        //var NAB = Console.ReadLine();
        //var N = NAB[0];
        //var B = NAB[2];
        //var A = NAB[1];

        //var vn = Console.ReadLine().Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => decimal.Parse(x)).ToList();
        //vn.Sort();
        //vn.Reverse();

        //decimal maxave = -1;
        //decimal before = -1;
        //decimal count = 0;
        //for (int i = 0; i < N; i++)
        //{
        //    if (before == vn[i])
        //    {

        //    }
        //    else
        //    {
        //        decimal num = 0;
        //        for (int j = 0; j <= i; j++)
        //        {
        //            num += vn[j];
        //        }
        //        if (maxave < num/(i + 1))
        //        {
        //            maxave = num / (i + 1);
        //            count = 1;
        //        }
        //        else if (maxave == num/(i + 1))
        //        {
        //            count++;
        //        }
        //    }


        //    before = vn[i];
        // 1
        //}
        #endregion



        var NM = Console.ReadLine().SplittedStringToInt32List();
        var N = NM[0];
        var M = NM[1];

        var edges = new List<Edge>();


        for (int i = 0; i < M; i++)
        {
            var abc = Console.ReadLine().SplittedStringToInt32List();
            var a = abc[0];
            var b = abc[1];
            var c = abc[2];

            edges.Add(new Edge() { Cost = -c, To = b - 1, From = a - 1 });
        }
        InitializeBellmanFord(N, M, edges);

        SearchShortestPath(0);
        var ans = ShortestDistances[N - 1];
        // InitializeBellmanFord(N, M, edges);

        SearchShortestPath2(0);
        if (Negatives[N - 1])
        {
            Console.WriteLine("inf");
        }
        else
        {
            Console.WriteLine(-ans);
        }

        Console.ReadLine();
        return;
        if (HasNegativeLoopFromSToG(0, N-1))
        {
            Console.WriteLine("inf");
        }
        else
        {
            SearchShortestPath(0);
            if (isfalse)
                Console.WriteLine("inf");
            else
                Console.WriteLine(-ShortestDistances[N-1]);
        }

        Console.ReadLine();
    }

    // s番目の頂点から各々の頂点への最短距離を求めるBellman-Ford

    public static void InitializeBellmanFord(int v, int e, IEnumerable<Edge> edges)
    {
        V = v;
        Edges = edges.ToArray();
        ShortestDistances = new long[V];
        E = e;
    }

    public class Edge
    {
        public int From;
        public int To;
        public int Cost;
    }

    public static Edge[] Edges;

    public static long[] ShortestDistances;

    public static int V;

    public static int E;

    public static bool isfalse = false;

    public static void SearchShortestPath(int s)
    {
        for (int i = 0; i < V; i++)
        {
            ShortestDistances[i] = LONGINF;
        }
        ShortestDistances[s] = 0;
        var vcount = 0;
        while (vcount != V - 1)
        {
            vcount++;
            var update = false;
            for (int i = 0; i < E; i++)
            {
                var e = Edges[i];
                if (ShortestDistances[e.From] != LONGINF && ShortestDistances[e.To] > ShortestDistances[e.From] + e.Cost)
                {
                    ShortestDistances[e.To] = ShortestDistances[e.From] + e.Cost;
                    update = true;
                }
            }
            //if (!update)
              //  break;
        }
    }
    public static bool[] Negatives;
    public static void SearchShortestPath2(int s)
    {

        Negatives = new bool[V];
        ShortestDistances[s] = 0;
        var vcount = 0;
        while (vcount != V)
        {
            vcount++;
            var update = false;
            for (int i = 0; i < E; i++)
            {
                var e = Edges[i];
                if (ShortestDistances[e.From] != LONGINF && ShortestDistances[e.To] > ShortestDistances[e.From] + e.Cost)
                {
                    ShortestDistances[e.To] = ShortestDistances[e.From] + e.Cost;
                    update = true;
                    Negatives[e.To] = true;
                }
            }
           // if (!update)
             //   break;
        }
    }

    // 負閉路をもつか？
    public static bool HasNegativeLoopAllgraph()
    {
        var d = new int[V];
        for (int i = 0; i < V; i++)
        {
            for (int j = 0; j < E; j++)
            {
                var e = Edges[j];
                if (d[e.To] > d[e.From] + e.Cost)
                {
                    d[e.To] = d[e.From] + e.Cost;
                    if (i == V - 1)
                    {
                        return true;
                    }
                }
            }
        }
        return false;
    }
    // 負閉路をもつか？（start固定）
    public static bool HasNegativeLoopFromS(int s)
    {
        var d = new int[V];
        for (int i = 0; i < V; i++)
        {
            d[i] = INF;
        }
        d[s] = 0;
        for (int i = 0; i < V; i++)
        {
            for (int j = 0; j < E; j++)
            {
                var e = Edges[j];
                if (d[e.From] != INF && d[e.To] > d[e.From] + e.Cost)
                {
                    d[e.To] = d[e.From] + e.Cost;
                    if (i == V - 1)
                        return true;
                }
            }
        }
        return false;
    }
    // 負閉路をもつか？(start,destination固定)
    public static bool HasNegativeLoopFromSToG(int s, int g)
    {
        var d = new long[V];
        for (int i = 0; i < V; i++)
        {
            d[i] = LONGINF;
        }
        d[s] = 0;
        for (int i = 0; i < 2 * V; i++)
        {
            for (int j = 0; j < E; j++)
            {
                var e = Edges[j];
                if (d[e.From] != LONGINF && d[e.To] > d[e.From] + e.Cost)
                {
                    d[e.To] = d[e.From] + e.Cost;
                    if (i >= V - 1 && e.To == g)
                        return true;
                }
            }
        }
        return false;
    }

    #region ライブラリ
    public static long ToInt64(this string str) => long.Parse(str);
    public static int ToInt32(this string str) => int.Parse(str);
    public static BigInteger ToBigInteger(this string str) => BigInteger.Parse(str);
    public static List<string> SplittedStringToList(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).ToList();
    public static List<int> SplittedStringToInt32List(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => int.Parse(x)).ToList();
    public static List<BigInteger> SplittedStringToBigInteger(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => BigInteger.Parse(x)).ToList();

    public const int INF = int.MaxValue / 2;

    public const long LONGINF = long.MaxValue / 2;
    #endregion
}