#include<stdio.h>
#include<string.h>
#include<stdbool.h>

#define N 100010
#define INF 0x3f3f3f3f

int n, m;
int dist[N];
int queue[N];
bool inQue[N];
int head, tail;
int h[N], cnt;

struct edge{
    int to;
    int next;
    int weight;
}E[N];

void add(int v, int w, int weight){
    E[++cnt].next = h[v];
    E[cnt].to = w;
    E[cnt].weight = weight;
    h[v] = cnt;
}

int spfa(){
    memset(dist, 0x3f, sizeof dist);
    dist[1] = 0;
    queue[++tail] = 1;  inQue[1] = true;

    while(head != tail){
        int t = queue[++head];
        inQue[t] = false;

        for(int i = h[t]; i != -1; i = E[i].next){
            int u = E[i].to;
            if(dist[t] + E[i].weight < dist[u]){
                dist[u] = dist[t] + E[i].weight;
                if(!inQue[u]){
                    queue[++tail] = u;
                    inQue[u] = true;
                }
            }
        }

    }

    if(dist[n] == INF)  return -1;
    else  return dist[n];
}


int main(){

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

    memset(h, -1, sizeof h);
    for(int i = 1; i <= m; i++){
        int a, b, w;
        scanf("%d %d %d", &a, &b, &w);
        add(a, b, w);
    }

    int res = spfa();

    if(res == -1) printf("impossible");
    else  printf("%d", res);

    return 0;
}