1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <algorithm>
 5 using namespace std;
 6 const int maxn = 5005;
 7 struct edge
 8 {
 9     int u,v,w;
10 };
11 edge e[maxn];
12 int pre[105];
13 int Find(int x)
14 {
15     return pre[x]==x?x:pre[x]=Find(pre[x]);
16 }
17 bool cmp(edge A,edge B)
18 {
19     return A.w<B.w;
20 }
21 int main()
22 {
23     int n,m;
24     while(scanf("%d %d",&n,&m)!=EOF&&(n||m))
25     {
26         for(int i=1;i<=m;i++)
27         {
28             scanf("%d %d %d",&e[i].u,&e[i].v,&e[i].w);
29         }
30         sort(e+1,e+m+1,cmp);
31         int count1 = 1;
32         int ans = 20000;
33         for(int i=1;i<=m;i++)
34         {
35             for(int k=1;k<=n;k++) pre[k] = k;
36             count1 = 1;
37             int minv = 20000;
38             int maxv = 0;
39             for(int j=i;j<=m;j++)
40             {
41                 int x = Find(e[j].u);
42                 int y = Find(e[j].v);
43                 if(x!=y)
44                 {
45                     count1++;
46                     pre[x] = y;
47                     maxv = max(maxv,e[j].w);
48                     minv = min(minv,e[j].w);
49                 }
50                 if(count1>=n)
51                 {
52                     break;
53                 }
54             }
55             if(count1>=n)
56             {
57                 ans = min(ans,maxv-minv);
58             }
59         }
60         if(ans!=20000) printf("%d\n",ans);
61         else printf("-1\n");
62     }
63     return 0;
64 }