#include<iostream>

 #include<cstdio>

 #include<cstring>

 #include<queue>

 using namespace std;

 struct edge{

     int to;

     int next;

     int len;

 }qwq[];

 queue<int>pq;

 int edge_cnt=,n,m,head[],in[],stk[],dist[];

 bool spfa()

 {

     memset(in,,sizeof(in));

     memset(stk,,sizeof(stk));

     memset(dist,-,sizeof(dist));

     dist[]=;

     while(!pq.empty())

     {

         pq.pop();

     }

     pq.push();

     in[]++;

     stk[]=;

     while(!pq.empty())

     {

         int qaq=pq.front();pq.pop();

         stk[qaq]=;

         for(int i = head[qaq];i!=-;i=qwq[i].next)

         {

             int v=qwq[i].to;

             if(dist[v]<dist[qaq]+qwq[i].len)

             {

                 dist[v]=dist[qaq]+qwq[i].len;

                 if(!stk[v])

                 {

                     pq.push(v);

                     in[v]++;

                     stk[v]=;

                     if(in[v]>n+){

                         return false;

                     }

                 }

             }

         }

     }

     return true;

 }

 void add(int x,int y,int z)

 {

     qwq[edge_cnt].to=y;

     qwq[edge_cnt].next=head[x];

     qwq[edge_cnt].len=z;

     head[x]=edge_cnt++;

 }

 int main()

 {

         scanf("%d%d",&n,&m);

            memset(head,-,sizeof(head));

          edge_cnt=;

         for(int i =  ; i < m ;i++)

         {

             int a,b,c;

              scanf("%d%d%d",&a,&b,&c);

             add(a,b,c);

          }

      if(!spfa())printf("-1\n");

     return ;

  }