#include <iostream>
#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 dist[N];
bool st[N];

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

int spfa()
{
    queue<int> q;
    q.push(1);

    memset(dist, 0x3f, sizeof dist);
    dist[1] = 0;
    st[1] = true;

    while (q.size())
    {
        int k = q.front(); q.pop();
        st[k] = false;

        for (int i = h[k]; i != -1; i = ne[i])
        {
            int j = e[i];
            if (dist[j] > dist[k] + w[i])
            {
                dist[j] = dist[k] + w[i];
                if (!st[j])
                {
                    st[j] = true;
                    q.push(j);
                }
            }
        }
    }

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

int main()
{
    memset(h, -1, sizeof h);

    cin >> n >> m;
    while (m -- )
    {
        int x, y, z;
        cin >> x >> y >> z;
        add(x, y, z);
    }

    int ans = spfa();
    if (ans == -1) puts("impossible");
    else cout << ans << endl;

    return 0;
}