import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.Queue;

//只要没有负环就可以用。用的邻接表。
public class Main {

    static int N = 100010;
    static int n, m,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]; 
    static boolean[] st=new boolean[N];

    private static void add(int a, int b, int c)
    {
        e[idx] = b; w[idx] = c; ne[idx] = h[a]; h[a] = idx ++ ;
    }

    private static int spfa()
    {
        Arrays.fill(dist,0x3f3f3f3f); //不能写成0x3f

        dist[1] = 0;

        Queue<Integer> q=new LinkedList<>();

        q.offer(1); 

        st[1] = true;

        while (q.size()>0)
        {
            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.offer(j);
                        st[j] = true;
                    }
                }
            }
        }

        return dist[n];
    }

    public static void main(String[] args) throws Exception{

        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        String s1[]=br.readLine().split(" ");

        n=Integer.parseInt(s1[0]);
        m=Integer.parseInt(s1[1]);

        Arrays.fill(h, -1);

        while (m -- >0)
        {
            int a, b, c;
            s1=br.readLine().split(" "); //第二次用，不用加[ ]
            a=Integer.parseInt(s1[0]);
            b=Integer.parseInt(s1[1]);
            c=Integer.parseInt(s1[2]);
            add(a, b, c);
        }

        int t = spfa();

        if (t == 0x3f3f3f3f) System.out.println("impossible");
        else System.out.println(t);
    }

}