import java.util.*;

public class Main {
    static int N = 100010;//边数M
    static int n,m;
    static int idx;
    static int[] h = new int[N];
    static int[] w = new int[N];
    static int[] e = new int[N];
    static int[] ne = new int[N];
    static int[] dist = new int[N];      //用于记录每一个点距离第一个点的距离
    static boolean[] st = new boolean[N];//用于记录该点的最短距离是否已经确定
    static int spfa(){
        for(int i=0;i<N;i++){
            dist[i]=0x3f3f3f;            //初始化距离  0x3f代表无限大
        }
        dist[1]=0;
        Queue<Integer> q = new LinkedList<>();
        q.add(1);
        st[1]=true;
        while(!q.isEmpty()){
           int t = q.poll();
           st[t]=false;
           for(int i=h[t];i!=-1;i=ne[i]){
               int j = e[i];
               if(dist[j]>dist[t]+w[i]){
                   dist[j]=dist[t]+w[i];
                   if(!st[j]){
                       q.add(j);
                       st[j]=true;
                   }
               }
           }
        }
        if(dist[n]==0x3f3f3f)   return -1; //如果第n个点路径为无穷大即不存在最低路径
        return dist[n];
    }
    static void add(int a,int b,int c){
        e[idx]=b;   w[idx]=c;   ne[idx]=h[a];   h[a]=idx++;
    }
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        n = sc.nextInt();
        m = sc.nextInt();
        for(int i=0;i<N;i++){
            h[i]=-1;
        }
        while(m-->0){
            int x = sc.nextInt();
            int y = sc.nextInt();
            int z = sc.nextInt();
            add(x,y,z);
        }
        if(spfa()==-1)  System.out.print("impossible");
        else System.out.print(spfa());
    }
}