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

const int N = 100010,M=100010;

int h[N],w[M],ne[M],e[M],idx;
int dist[N];
bool st[N]; //表示是否在队列中
int n,m;

void add(int a,int b,int c)
{
    w[idx]=c,e[idx]=b,ne[idx]=h[a],h[a]=idx++;
}

int spfa()
{
    memset(dist,0x3f,sizeof dist);
    queue<int> q;
    q.push(1);
    dist[1]=0;
    while(q.size())
    {
        int t=q.front();
        q.pop();
        st[t]=false;
        for(int i=h[t];i!=-1;i=ne[i])
        {
            int x=e[i];
            if(dist[x]>dist[t]+w[i])
            {
                dist[x]=dist[t]+w[i];
                q.push(x);
                st[x]=true;
            }
        }
    }
    if(dist[n]==0x3f3f3f3f) return -1;
    return dist[n];
}

int main()
{
    cin>>n>>m;
    memset(h,-1,sizeof h);
    for(int i=0;i<m;i++)
    {
        int a,b,c;
        scanf("%d%d%d",&a,&b,&c);
        add(a,b,c);
    }
    int flag=spfa();
    if(flag==-1)
        puts("impossible");
    else cout<<flag;
    return 0;
}