1 #include<stdio.h>
 2 #include<string.h>
 3 const int N=1000+5;
 4 struct EDGE{
 5     int v,w,next;
 6 }edge[N*N*2];
 7 int first[N],low[N],dfn[N],vis[N];
 8 int g,cnt,sum,ans;
 9 int min(int a,int b)
10 {
11     return a<b?a:b;
12 }
13 void AddEdge(int u,int v,int w)
14 {
15     edge[g].v=v;
16     edge[g].w=w;
17     edge[g].next=first[u];
18     first[u]=g++;
19 }
20 void Tarjan(int u,int fa)
21 {
22     int i,v;
23     low[u]=dfn[u]=++cnt;
24     for(i=first[u];i!=-1;i=edge[i].next)
25     {
26         v=edge[i].v;
27     //    if(v==fa)
28     //        continue;
29         if(i==(fa^1))
30             continue;
31         if(!dfn[v])
32         {
33             Tarjan(v,i);
34             low[u]=min(low[u],low[v]);
35             if(low[v]>dfn[u])
36             {
37                 if(edge[i].w<ans)
38                     ans=edge[i].w;
39             }
40         }
41         else
42             low[u]=min(low[u],dfn[v]);
43     }
44 }
45 void dfs(int u)
46 {
47     if(vis[u])
48         return ;
49     vis[u]=1;
50     sum++;
51     for(int i=first[u];i!=-1;i=edge[i].next)
52         dfs(edge[i].v);
53 }
54 int main()
55 {
56     int i,n,m,u,v,w;
57     while(scanf("%d%d",&n,&m))
58     {
59         if(n==0&&m==0)
60             break;
61         g=cnt=0;
62         memset(first,-1,sizeof(first));
63         memset(dfn,0,sizeof(dfn));
64         memset(vis,0,sizeof(vis));
65         while(m--)
66         {
67             scanf("%d%d%d",&u,&v,&w);
68             AddEdge(u,v,w);
69             AddEdge(v,u,w);
70         }
71         sum=0;
72         dfs(1);
73         if(sum<n)
74         {
75             printf("0\n");
76             continue;
77         }
78         ans=1000000;
79     //    for(i=1;i<=n;i++)
80     //        if(!dfn[i])
81     //            Tarjan(i,0);
82         Tarjan(1,-1);
83         if(ans==1000000)
84             printf("-1\n");
85         else
86         {
87             if(ans==0)
88                 ans=1;
89             printf("%d\n",ans);
90         }
91     }
92     return 0;
93 }