#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
#include<iostream>
#include<algorithm>
#include<bitset>
#include<functional>
using namespace std;
typedef long long LL;
const int maxn = 5005;
const int INF = 0x7FFFFFFF;
int T, n, m, fa[maxn];

struct point
{
    int x, y, cost;
    void read(){ scanf("%d%d%d", &x, &y, &cost); }
    bool operator<(const point&a)const{ return cost < a.cost; };
}a[maxn * 10];

int get(int x)
{
    return fa[x] == x ? fa[x] : fa[x] = get(fa[x]);
}

int main(){
    scanf("%d", &T);
    while (T--)
    {
        scanf("%d%d", &n, &m);
        for (int i = 0; i < m; i++) a[i].read();
        sort(a, a + m);
        int ans = INF;
        for (int i = 0; i < m; i++)
        {
            int cnt = n - 1, res;
            for (int j = 1; j <= n; j++) fa[j] = j;
            for (int j = i; j < m; j++)
            {
                int fx = get(a[j].x), fy = get(a[j].y);
                if (fx == fy) continue; else fa[fx] = fy;
                if (!(--cnt)) { res = a[j].cost - a[i].cost; break; }
            }
            if (cnt) break;
            ans = min(ans, res);
        }
        if (ans == INF) printf("-1\n"); else printf("%d\n", ans);
    }
    return 0;
}