1 //Accepted    200 KB    94 ms
  2 //kruskal
  3 #include <cstdio>
  4 #include <cstring>
  5 #include <cmath>
  6 #include <algorithm>
  7 using namespace std;
  8 
  9 const int MAXN = 105;
 10 const int MAXM = MAXN*MAXN;
 11 const int inf=10000000;
 12 
 13 int min(int a,int b)
 14 {
 15     return a>b?b:a;
 16 }
 17 int max(int a,int b)
 18 {
 19     return a>b?a:b;
 20 }
 21 
 22 struct node
 23 {
 24     int u,v,c;
 25     node(int u,int v,int c):u(u),v(v),c(c)
 26     {
 27 
 28     }
 29     node()
 30     {
 31 
 32     }
 33 }p[MAXM];
 34 int e;
 35 
 36 void addnode(int u,int v,int c)
 37 {
 38     p[e++]=node(u,v,c);
 39 }
 40 int cmp(node x,node y)
 41 {
 42     return x.c<y.c;
 43 }
 44 int f[MAXN];
 45 int n,m;
 46 void build()
 47 {
 48     for (int i=0;i<=n;i++)
 49     {
 50         f[i]=i;
 51     }
 52 }
 53 int find(int x)
 54 {
 55     if (x==f[x]) return x;
 56     return f[x]=find(f[x]);
 57 }
 58 void union_set(int x,int y)
 59 {
 60     int fx=find(x),fy=find(y);
 61     if (fx!=fy)
 62     {
 63         f[fy]=fx;
 64     }
 65 }
 66 void kruskal()
 67 {
 68     int ans=inf;
 69     int cnt;
 70     int tmin,tmax;
 71     sort(p,p+e,cmp);
 72     for (int i=0;i<m;i++)
 73     {
 74         build();
 75         cnt=0;
 76         tmax=-1;
 77         tmin=inf;
 78         for (int j=i;j<e;j++)
 79         {
 80             int fx=find(p[j].u);
 81             int fy=find(p[j].v);
 82             if (fx!=fy)
 83             {
 84                 union_set(fx,fy);
 85                 tmax=max(tmax,p[j].c);
 86                 tmin=min(tmin,p[i].c);
 87                 cnt++;
 88                 if (cnt==n-1) break;
 89             }
 90         }
 91         if (cnt==n-1)
 92         ans=min(ans,tmax-tmin);
 93     }
 94     if (ans==inf) ans=-1;
 95     printf("%d\n",ans);
 96 }
 97 int main()
 98 {
 99     while (scanf("%d%d",&n,&m),n+m)
100     {
101         e=0;
102         int u,v,c;
103         for (int i=0;i<m;i++)
104         {
105             scanf("%d%d%d",&u,&v,&c);
106             addnode(u,v,c);
107         }
108         kruskal();
109     }
110     return 0;
111 }