#include<iostream>
#include<algorithm>
#include<cstring>
#include<queue>

using namespace std;
typedef pair<int, int>PII;
const int N = 250010;
int n, m;
int h[N], w[N], e[N], ne[N], idx;
int dist[N];
bool st[N];
void add(int a, int b, int c)
{ 
    e[idx] = b;
    w[idx] = c;
    ne[idx] = h[a];
    h[a] = idx++;
}
int sofa()
{
    queue<int>q;
    memset(dist, 0x3f, sizeof (dist));
    dist[1] = 0;
    st[1] = true;
    q.push(1);
    while(q.size())
    {
        auto t =q.front();
        q.pop();
        st[t] = false;

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

    if(dist[n] == 0x3f3f3f3f)
    return -1;
    return dist[n];
}
int main()
{
    cin >> n >> m;
    memset(h, -1 , sizeof h);
    while(m--)
    {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
    }
    if(sofa() == -1)
    cout << "impossible" << endl;
    else
    cout << sofa()<<endl;
}