1 #include<iostream>
 2 #include<algorithm>
 3 #include<cstdio>
 4 #include<cstring>
 5 #include<cstdlib>
 6 #include<vector>
 7 #include<map>
 8 using namespace std;
 9 const int maxn=1007;
10 const int inf=0x3f3f3f3;
11 struct node
12 {
13     int to,cost,next;
14 };
15 node edge[maxn*maxn];
16 int head[maxn],n,m,num,ans,sum,pre[maxn],child,low[maxn];
17 void addedge(int u,int v,int c)
18 {
19     edge[num].to=v;edge[num].cost=c;edge[num].next=head[u];
20     head[u]=num++;
21     edge[num].to=u;edge[num].cost=c;edge[num].next=head[v];
22     head[v]=num++;
23 }
24 void tarjan(int u,int fa)
25 {
26     child++;
27     low[u]=pre[u]=sum++;
28     for(int i=head[u];i!=-1;i=edge[i].next)
29     {
30         int v=edge[i].to;
31         if(i==(fa^1)) continue;
32         if(!pre[v]){
33             tarjan(v,i);
34             low[u]=min(low[u],low[v]);
35             if(pre[u]<low[v]){
36                 ans=min(ans,edge[i].cost);
37             }
38         }
39         else low[u]=min(low[u],low[v]);
40     }
41 }
42 int main()
43 {
44     int a,b,c;
45     while(~scanf("%d%d",&n,&m)){
46         if(n==0&&m==0) break;
47         memset(head,-1,sizeof(head));
48         memset(pre,0,sizeof(pre));
49         memset(low,0,sizeof(low));
50         ans=inf;child=0;sum=1;
51         for(int i=1;i<=m;i++)
52         {
53             scanf("%d%d%d",&a,&b,&c);
54             addedge(a,b,c);
55         }
56         tarjan(1,-1);
57         if(child<n) printf("0\n");
58         else if(ans==inf) printf("-1\n");
59         else if(ans==0) printf("1\n");
60         else printf("%d\n",ans);
61     }
62     return 0;
63 }