#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;

const int N = 1e5 + 10;
int e[N], h[N], ne[N], idx, w[N];
int q[N], dist[N];
int hh = 0, tt = -1;
int n, m;
bool st[N];


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

int main() {
    memset(h, -1, sizeof h);
    memset(dist, 0x3f, sizeof dist);
    cin >> n >> m;
    for (int i = 0; i < m; i ++ ) {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
    }
    dist[1] = 0;
    q[++ tt] = 1;
    st[1] = true;
    while (hh <= tt) {
        auto t = q[hh ++ ];
        st[t] = false;
        for (int i = h[t]; i != -1; i = ne[i]) {
            int j = e[i], we = w[i];
            if (dist[j] > dist[t] + we) {
                dist[j] = dist[t] + we;
                if (!st[j]) {
                    q[++ tt] = j;
                    st[j] = true;
                }
            }
        }
    }
    if (dist[n] == 0x3f3f3f3f) cout << "impossible" << endl;
    else cout << dist[n] << endl;
    return 0;
}