//https://www.acwing.com/problem/content/853/
// Created by guany on 25-1-9.
#include <bits/stdc++.h>
#include <queue>
using namespace std;
const int N=1e5+10;
//使用邻接表存图
int h[N],ne[N],e[N],w[N],idx;
//spfa“松弛”算法借助队列来实现
queue<int> q;
int st[N];//st记录节点是否还在队列中
int dist[N];//存储节点n到源节点的最短距离
void add(int x,int y,int z)
{
  w[idx]=z;
  ne[idx]=h[x];
  e[idx]=y;
  h[x]=idx++;
}
void spfa()
{
  st[1]=1;
  q.push(1);
  //源节点到自身的距离为0
  dist[1]=0;
  while(!q.empty()){
    int u=q.front();
    q.pop();
    st[u]=0;
    for(int i=h[u];~i;i=ne[i]){
      if(dist[e[i]]>dist[u]+w[i]){
        if(!st[e[i]]){
          st[e[i]]=1;
          q.push(e[i]);
          }
          dist[e[i]]=dist[u]+w[i];
      }
      }
    }
  }
int main()
{
  memset(dist,0x3f,sizeof(dist));
  memset(h,-1,sizeof(h));
  int n,m;
  cin>>n>>m;
  while(m--)
    {
      int x,y,z;
      cin>>x>>y>>z;
      add(x,y,z);
    }
  spfa();
  if(dist[n]==0x3f3f3f3f) cout<<"impossible"<<endl;
  else cout<<dist[n]<<endl;
  return 0;
}