#include <bits/stdc++.h>
using namespace std;

#define int long long

void solve()
{
    int k;
    cin >> k;
    vector<pair<int, int>> p;
    for (int i = 0; i < k; i++)
    {
        int x;
        cin >> x;
        while (x--)
        {
            int v;
            cin >> v;
            p.emplace_back(v, i);
        }
    }

    sort(p.begin(), p.end());
    int n = p.size();

    int sz = 0;
    map<int, int> cnt;
    int r = 0;
    int ans = 1e18;
    for (int l = 0; l < n; l++)
    {
        while (r < n && sz < k)
        {
            cnt[p[r].second]++;
            if (cnt[p[r].second] == 1)
                sz++;
            r++;
        }
        if (sz < k)
            break;

        ans = min(ans, p[r - 1].first - p[l].first);
        cnt[p[l].second]--;
        if (cnt[p[l].second] == 0)
            sz--;
    }
    cout << ans << '\n';
}

signed main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);

    int t;
    cin >> t;
    while (t--)
    {
        solve();
    }

    return 0;
}