#include <bits/stdc++.h>
using namespace std;

const int N = 100010;

int n, m;
int dist[N];
bool st[N];
vector<pair<int,int>> g[N];
void add(int a, int b, int c)
{
    g[a].push_back({b,c});
}

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 = 0; i<g[t].size(); i++)
        {
            int j = g[t][i].first;
            if (dist[j] > dist[t] + g[t][i].second)
            {
                dist[j] = dist[t] + g[t][i].second;
                if (!st[j])
                {
                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }

    return dist[n];
}

int main()
{
    scanf("%d%d", &n, &m);


    while (m -- )
    {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        add(a, b, c);
    }

    int t = spfa();

    if (t == 0x3f3f3f3f) puts("impossible");
    else printf("%d\n", t);

    return 0;
}