#include<stdio.h>
#include<malloc.h>
#include<queue>
#include<string.h>
using namespace std;
typedef struct node
{
    int id;
    int distance;
    struct node *next;
}Node,*LNode;
int n,m;
LNode array[100010];//鄰接鏈表
int in_queue[100010];//記錄一個點是否在隊列中
int between_distance[100010];//記錄起始點到兩個點之間的距離
queue<int> q;//建立一個隊列

void init_array()
{
    for(int i = 0;i <= n;++i)
    {
        array[i] = (LNode)malloc(sizeof(Node));
        array[i]->id = i;
        array[i]->distance = 0;
        array[i]->next = NULL;
    }
}

void insert_node(LNode header,LNode &insert_node)
{
    while(header->next != NULL)
        header = header->next;
    insert_node->next = header->next;
    header->next = insert_node;
}

//任何兩點之間的距離設置為-1，表示正無窮
void init_distance()
{
    for(int i = 0;i <= n;++i)
    {
        between_distance[i] = -1;
    }
    //第一個結點到自身為0
    between_distance[1] = 0;
}

//松弛
void relax(int index)
{
    if(between_distance[index] == -1)
        return ;
    LNode header = array[index]->next;
    while(header != NULL)
    {
        if(between_distance[header->id] > between_distance[index] + header->distance || between_distance[header->id] == -1)
        {
            between_distance[header->id] = between_distance[index] + header->distance;
            if(!in_queue[header->id])
            {
                q.push(header->id);
                in_queue[header->id] = 1;
            }
        }
        header = header->next;
    }
}

void SPFA()
{
    //第一個點入隊列
    q.push(1);
    in_queue[1] = 1;
    int index = 0;
    while(!q.empty())
    {
        index = q.front();
        q.pop();
        in_queue[index] = 0;
        relax(index);
    }
}

int main()
{
    int x,y,distance;
    scanf("%d%d",&n,&m);
    init_array();
    for(int i = 0;i < m;++i)
    {
        scanf("%d%d%d",&x,&y,&distance);
        LNode node = (LNode)malloc(sizeof(Node));
        node->id = y;
        node->distance = distance;
        insert_node(array[x],node);
        node = (LNode)malloc(sizeof(Node));
        node->id = x;
        node->distance = distance;
        insert_node(array[y],node);
    }
    init_distance();
    memset(in_queue,0,sizeof(in_queue));
    SPFA();
    if(between_distance[n] == -1)
    {
        printf("-1\n");
    }
    else
        printf("%d\n",between_distance[n]);
    return 0;
}