#include <bits/stdc++.h>
using namespace std;
int n,m;
const int N  = 1e5+10;
int e[N],h[N],ne[N],idx,w[N];
int dist[N],st[N];
void add(int a,int b,int c)
{
    e[idx] = b,ne[idx] = h[a],w[idx] = c,h[a] = idx++;
}
int spfa()
{
    memset(dist,0x3f,sizeof dist);
    dist[1] = 0;
    st[1] = true;
    queue<int> q;
    q.push(1);
    while(!q.empty())
    {
            int 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;
                    }
                }
            }
    }
    return dist[n];
}
int main()
{
    memset(h,-1,sizeof h);
    cin>>n>>m;
    while( m -- )
    {
        int a,b,c;
        cin>>a>>b>>c;
        add(a,b,c);
    }
    int t = spfa();
    if(t == 0x3f3f3f3f)
        cout<<"impossible";
    else
        cout<<t;
}