#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
#define Maxn 15010
using namespace std;

struct line{
    int u,v,w;
    line(){}
    line(int uu,int vv,int ww):u(uu),v(vv),w(ww){}
    bool operator<(const line &a)const{
        return w<a.w;
    }
}edge[Maxn];
int fa[2010];
void init(int n){
    for(int i=1;i<=n;i++)
        fa[i]=i;
}
int findset(int x){
    return fa[x]==x?x:(fa[x]=findset(fa[x]));
}
void unionset(int a,int b){
    fa[a]=b;
}
int kruskal(int n,int tot,int fr){
    int cnt=0,ans=edge[fr].w;
    for(int i=fr;i<tot;i++){
        int a=findset(edge[i].u),b=findset(edge[i].v);
        if(a!=b){
            //ans+=edge[i].w;
            unionset(a,b);
            if(++cnt==n-1){
                ans=edge[i].w-ans;
                break;
            }
        }
    }
    if(cnt!=n-1) return -1;
    return ans;
}
int main()
{
    int t,n,m,u,v,w;
    cin>>t;
    while(t--){
        cin>>n>>m;
        for(int i=0;i<m;i++){
            scanf("%d%d%d",&u,&v,&w);
            edge[i]=line(u,v,w);
        }
        init(n);
        sort(edge,edge+m);
        //for(int i=0;i<m;i++) cout<<edge[i].w<<"+++"<<endl;
        int ans=kruskal(n,m,0);
        //cout<<ans<<"++++++"<<endl;
        if(ans==-1) cout<<-1<<endl;
        else{
            for(int i=0,j;i<m;i=j){
                j=i;
                while(j<m&&edge[j].w==edge[i].w) j++;
                init(n);
                int tmp=kruskal(n,m,i);
                //cout<<tmp<<"***"<<j<<endl;
                if(tmp!=-1) ans=min(ans,tmp);
            }
            cout<<ans<<endl;
        }
    }
    return 0;
}