1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 using namespace std;
 5 struct edge{
 6     int y,ne,v;
 7 }edge[200010];
 8 int d1[5010],d2[5010],n,m,head[5010],len=0,queue[200010];
 9 bool vis[5010];
10 inline void addedge(int x,int y,int v)
11 {edge[++len].y=y;    edge[len].v=v;    edge[len].ne=head[x];    head[x]=len;}
12 void init()
13 {
14     memset(head,-1,sizeof(head));
15     scanf("%d%d",&n,&m);
16     int x,y,v;
17     for (int i=1;i<=m;i++)
18     {
19         scanf("%d%d%d",&x,&y,&v);
20         addedge(x,y,v);
21         addedge(y,x,v);
22     }
23 }
24 
25 void spfa()
26 {
27     memset(d1,0x3f,sizeof(d1));
28     memset(d2,0x3f,sizeof(d2));
29     memset(vis,false,sizeof(vis));
30     int Head=0,tail=1;
31     queue[1]=1;    d1[1]=0;    vis[1]=true;
32     while (Head<tail)
33     {
34         int tn=queue[++Head];
35         int te=head[tn];
36         vis[tn]=false;
37         for (int i=te;i!=-1;i=edge[i].ne)
38         {
39             int temp=edge[i].y;
40             if (d1[temp]>d1[tn]+edge[i].v)
41             {
42                 d2[temp]=min(d1[temp],d2[tn]+edge[i].v);
43                 d1[temp]=d1[tn]+edge[i].v;
44                 if (!vis[temp])
45                 {
46                     vis[temp]=true;
47                     queue[++tail]=temp;
48                 }
49             }
50             else
51             {
52                 if (d1[temp]==d1[tn]+edge[i].v)
53                 {
54                     d2[temp]=min(d2[tn]+edge[i].v,d2[temp]);
55                     if (!vis[temp])
56                     {
57                         vis[temp]=true;
58                         queue[++tail]=temp;
59                     }
60                 }
61                 else if (d2[temp]>d1[tn]+edge[i].v)
62                 {
63                     d2[temp]=min(d1[tn]+edge[i].v,d2[temp]);
64                     if (!vis[temp])
65                     {
66                         vis[temp]=true;
67                         queue[++tail]=temp;
68                     }
69                 }
70             }
71         }
72     }
73     printf("%d\n",d2[n]);
74 }
75 
76 int main()
77 {
78     //freopen("add.in","r",stdin);
79     //freopen("add.out","w",stdout);
80     init();
81     spfa();
82     return 0;
83 }