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

using namespace std;

const int N = 100001;
int ne[N],e[N],h[N],w[N],idx;
int dis[N];
bool p[N];
int n,m;

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

int spa(){
    queue<int> qq;
    memset(dis,0x3f,sizeof dis);
    dis[1] = 0;
    p[1] = true;
    qq.push(1);
    while(qq.size()){
        auto f = qq.front();
        qq.pop();
        p[f]  = false;
        for(int i = h[f]; i != -1; i = ne[i]){
            int j = e[i];
            if(dis[j] > dis[f] + w[i]){
                dis[j] = dis[f] + w[i];
                if(p[j] == false){
                    p[j] = true;
                    qq.push(j);
                }
            }
        }
    }
    return dis[n];
}

int main(){
    cin >> n >> m;
    memset(h,-1,sizeof h);
    for(int i = 1; i <= m; i ++){
        int a,b,c;
        cin >> a >> b >> c;
        add(a,b,c);
    }
    int res = spa();
    if(res >= 0x3f3f3f) cout << "impossible" << endl;
    else cout << res << endl;
}