#include <bits/stdc++.h>
using namespace std;
const int maxn = 100005;

int cnt;
int head[maxn];
int dis[maxn];
bool st[maxn];  //用于记录节点是否在更新队列当中

struct  Edge {

    int to, next, wt;
}edges[maxn];

void add(int u, int v, int w) {

    edges[++cnt].to = v;
    edges[cnt].wt = w;
    edges[cnt].next = head[u];
    head[u] = cnt;
}

int spfa(int s, int d) {

    memset(dis, 0x3f, sizeof dis);
    memset(st, 0, sizeof st);
    queue<int> q;   //更新队列

    dis[s] = 0; st[s] = true;  q.push(s);

    while (q.size()) {

        int f = q.front();
        q.pop();
        st[f] = false;

        for (int e = head[f]; e != 0; e = edges[e].next) {

            int j = edges[e].to, w = edges[e].wt;
            if (dis[j] > dis[f] + w) {

                dis[j] = dis[f] + w;
                if (!st[j]) {

                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }

    if (dis[d] < 0x3f3f3f3f) {

        return dis[d];
    }
    else {

        return -1;
    }
}

int main() {

#ifndef ONLINE_JUDGE
    freopen("D:/VS CODE/C++/in.txt", "r", stdin);
    freopen("D:/VS CODE/C++/out.txt", "w", stdout);
#endif
    memset(head, -1, sizeof head);
    int n, m;
    cin >> n >> m;

    for (int i = 0;i < m;++i) {

        int u, v, w;
        scanf("%d %d %d", &u, &v, &w);
        add(u, v, w);
    }

    int t = spfa(1, n);

    if (t == -1) 
        cout << "impossible" << endl;
    else {

        cout << t;
    }

    fclose(stdin);
    fclose(stdout);
    return 0;
}