//spfa算法求单源最短路，能解决负权问题
//背模板
#include<bits/stdc++.h>
using namespace std;
int n,m;
#define inf 0x7ffffff
const int N = 100010;
int h[N],to[N],w[N],nxt[N];
int cnt;
int dist[N],vis[N];
void addedge(int a,int b,int val)
{
    nxt[++cnt] = h[a];
    h[a] = cnt;
    to[cnt] = b;
    w[cnt] = val;
}
void spfa()
{
    for(int i=1;i<=n;i++) dist[i] = inf;
    dist[1] = 0;
    queue<int> q;
    q.push(1);
    vis[1] = 1;

    while(!q.empty())
    {
        int now = q.front();
        q.pop();

        vis[now] = 0;

        for(int i=h[now];i;i=nxt[i])
        {
            int j = to[i];
            if(dist[j]>dist[now] + w[i])
            {
                dist[j] = dist[now] + w[i];
                if(vis[j]==0)
                {
                    q.push(j);
                    vis[j] = 1;
                }
            }
        }
    }
}
int main()
{
    cin>>n>>m;
    int x,y,z;
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d%d",&x,&y,&z);
        addedge(x,y,z);
    }
    spfa();
    if(dist[n]==inf) cout<<"impossible";
    else cout<<dist[n];
    return 0;
}