#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;

const int N = 1e5 + 10;
int h[N], e[N], ne[N], w[N], idx;
int n, m;
int dis[N], 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(dis, 0x3f, sizeof(dis));
    dis[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 (dis[j] > dis[t] + w[i]) {
                dis[j] = dis[t] + w[i];
                if (!st[j]) {
                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }
    return dis[n];
}

int main() {
    cin >> n >> m;
    memset(h, -1, sizeof(h));

    for (int i = 0; i < m; i++) {
        int a, b, c;
        scanf("%d %d %d", &a, &b, &c);
        add(a, b, c);
    }

    int res = spfa();
    if (res == 0x3f3f3f3f) cout << "impossible" << endl;
    else cout << res << endl;


    return 0;
}