#include <cstring>
#include <iostream>
#include <algorithm>
#include <queue>
using namespace std;
const int N = ;
int n, m;
int h[N], w[N], e[N], ne[N], idx;
int dist[N];
bool st[N];//标记数字是否在队列中,防止存重复的点
void add(int a, int b, int c) {
e[idx] = b, w[idx] = c, ne[idx] = h[a], h[a] = idx ++ ;
}
int spfa() {
memset(dist, 0x3f, sizeof dist);
dist[] = ;
queue<int> q;//存储所有待更新的点
q.push();
st[] = true;//标记已经放进去了
while (q.size()) {
int t = q.front();
q.pop();
st[t] = false;//表示不在队列中了
for (int i = h[t]; i != -; i = ne[i]) {//更新t的所有临边
int j = e[i];
if (dist[j] > dist[t] + w[i]) {
dist[j] = dist[t] + w[i];
if (!st[j]) {//如果不在队列中,再加进去
q.push(j);
st[j] = true;//标记
}
}
}
}
if (dist[n] == 0x3f3f3f3f) return -;
return dist[n];
}
int main() {
scanf("%d%d", &n, &m);
memset(h, -, sizeof h);
while (m -- ) {
int a, b, c;
scanf("%d%d%d", &a, &b, &c);
add(a, b, c);
}
int t = spfa();
if (t == -) puts("impossible");
else printf("%d\n", t);
return ;
}