#include <iostream>
#include <map>
#include <algorithm>
#include <stack>
#include <string>
#include <set>
#include <cmath>
#include <queue>
using namespace std;
#define rep(i,j,k) for(int i=j;i<=k;i++)
#define repm(i,j,k) for(int i=j;i>=k;i--)
#define mp make_pair
typedef long long ll;

struct DisjoinSet{
    std::vector<int> father,rank;
    DisjoinSet(int n) : father(n),rank(n) {
        for(int i=0;i<n;i++)
        father[i]=i;
    }
    int find(int v){
        return father[v]=(father[v]==v)?v:find(father[v]);
    }
    void merge(int x,int y){
        int a=find(x),b=find(y);
        if(rank[a]<rank[b])
        father[a]=b;
        else{
            father[b]=a;
            if(rank[b]==rank[a])
            ++rank[a];
        }
    }
};

struct edge{
    int s,t,c;
}e[15010];

bool cmp(edge a,edge b)
{
    return a.c<b.c;
}

int t,n,m;
int main()
{
    cin>>t;
    while(t--)
    {
        cin>>n;
        DisjoinSet s=DisjoinSet(n);
        cin>>m;
        rep(i,1,m)
        scanf("%d%d%d",&e[i].s,&e[i].t,&e[i].c),e[i].s--,e[i].t--;
        sort(e+1,e+1+m,cmp);
        int cnt=0;
        int mark=0;
        repm(i,m,1)
        {
            if(s.find(e[i].s)!=s.find(e[i].t))
            s.merge(e[i].s,e[i].t),cnt++;
            if(cnt==n-1)
            {
                mark=i;
                break;
            }
        }
        if(cnt!=n-1)
        cout<<"-1"<<endl;
        else
        {
            int cost=0x7f7f7f7f;
            rep(i,1,mark)
            {
                int curcost=0;
                cnt=0;
                s=DisjoinSet(n);
                rep(j,i,m)
                {
                    if(s.find(e[j].s)!=s.find(e[j].t))
                    s.merge(e[j].s,e[j].t),cnt++;
                    curcost=e[j].c-e[i].c;
                    if(curcost>cost)
                    break;
                    if(cnt==n-1)
                    {
                        if(curcost<cost)
                        cost=curcost;
                        break;
                    }
                }
            }
            cout<<cost<<endl;
        }
    }
}