#include<queue>
#include<cstring>
#include<iostream>
#include<algorithm>

using namespace std;

const int N = 100010, M = 2 * N;

int n, m;
int h[N], w[M], e[M], ne[M], idx;
int dist[N];
bool st[N];

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

int spfa() {
    memset(dist, 0x3f, sizeof dist);
    dist[1] = 0;

    queue<int> q;
    q.push(1);
    st[1] = true;

    while(q.size()) {
        int  t = q.front();
        q.pop();
        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.push(j);
                    st[j] = true;
                }
            }
        }
    }

    if(dist[n] == 0x3f3f3f3f) return -1;
    return dist[n];
}

int main() {
    cin >> n >> m;

    memset(h, -1, sizeof h);

    while(m --) {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
    }
    int t = spfa();
    if(t == -1) puts("impossible");
    else cout << t << endl;
    return 0;
}