#include <cstdio>
#include <cstring>
#include <algorithm>
#include <climits>
using namespace std;

const int N = 2005;
const int M = 15005;

struct Edge {
    int a, b, c;
} E[M];

bool cmp(Edge P, Edge Q) {
    return P.c < Q.c;
}

struct DSU {
    int fa[N];
    void init(int n = N - 1) {
        for(int i = 1; i <= n; i++) {
            fa[i] = i;
        }
    }
    int find(int x) {
        if(x == fa[x]) return x;
        return fa[x] = find(fa[x]);
    } 
    void merge(int i, int j) {
        i = find(i);
        j = find(j);
        fa[i] = j;
    }
    bool same(int i, int j) {
        return find(i) == find(j);
    }
} D;

int solve(int n, int m, int i) {
    D.init(n);
    int j = i;
    int cnt = 0;
    int last = -1;
    for(; i < m && cnt < n - 1; i++) {
        if(D.same(E[i].a, E[i].b)) continue;        
        D.merge(E[i].a, E[i].b);
        cnt++;
        last = i;
    }
    if(cnt == n - 1) {
        return E[last].c - E[j].c;
    } else {
        return -1;
    }
}
int main() {
    //freopen("in.txt", "r", stdin);
    int T, n, m;
    scanf("%d", &T);
    while(T--) {
        scanf("%d%d", &n, &m);
        for(int i = 0; i < m; i++) {
            scanf("%d%d%d", &E[i].a, &E[i].b, &E[i].c);
        }
        sort(E, E + m, cmp);
        int ans = INT_MAX;
        for(int i = 0; i < m; i++) {
            int ret = solve(n, m, i);
            if(~ret) {
                ans = min(ans, ret);
            } else {
                break;
            }
        }
        if(ans == INT_MAX) {
            printf("-1\n");
        } else {
            printf("%d\n", ans);
        }
    }
    return 0;
}