#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <map>
#include <stack>
#include <vector>
#include <memory.h>
#include <set>
#include <string.h>
#include <list>
#include <string>
#include <math.h>
#include <stdlib.h>
#include <queue>
#include <time.h>
#define ll long long
#define forn(i,n) for(int i=0;i<n;++i)
#define rep(i,n) for(int i=1;i<=n;++i)
#define repc(i,l,r) for(int i=l;i<=r;++i)
#define mp(a,b) make_pair(a,b)
#define llf long double
#define pii pair<int,int>
using namespace std;
int dsu[2005];
int n, m;
struct Road{
    int u, v, w;
    void read(){
        scanf("%d%d%d", &u, &v, &w);
    }
}rd[15005];
int cmp(const Road & a,const Road &b){
    return a.w < b.w;
}
int getgp(int u){
    if (!dsu[u]) return u;
    return dsu[u] = getgp(dsu[u]);
}
int cnt;
void bind(int u,int v){
    u = getgp(u);
    v = getgp(v);
    if (u != v){
        --cnt;
        dsu[v] = u;
    }
}
void solve(){
    cin >> n >> m;
    forn(i, m) rd[i].read();
    sort(rd, rd + m, cmp);
    int lf=0, rg=0;
    int res = 2e9 + 5;
    while (rg < m){
        rg = lf;
        cnt = n - 1;
        memset(dsu, 0, sizeof(dsu));
        while (rg < m&&cnt){
            bind(rd[rg].u, rd[rg].v);
            ++rg;
        }
        if (cnt) break;
        while (rg < m&&rd[rg].w == rd[rg].w - 1)++rg;
        cnt = n - 1;
        memset(dsu, 0, sizeof(dsu));
        lf = rg;
        while (cnt){
            --lf;
            bind(rd[lf].u, rd[lf].v);
        }
        res = min(res, rd[rg - 1].w - rd[lf].w);
        ++lf;
    }
    if (res == 2e9 + 5){
        res = -1;
    }
    cout << res << endl;
}
int main(){
//freopen("abc.txt", "r", stdin);
    int T;
    cin >> T;
    while (T--){
        solve();
    }
    return 0;
}