#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define pb push_back
#define eb emplace_back
#define all(x) (x).begin(),(x).end()
#define F first
#define S second


void solve(){
    int k;
    cin >> k;
    vector<vector<int>> sets(k);
    for(auto &st : sets){
        int n;
        cin >> n;
        st.assign(n, 0);
        for(auto &x : st) cin >> x;
        sort(all(st));
        reverse(all(st));
    }

    set<pair<int, int>> tops;
    for(int i = 0; i < k; i++){
        tops.insert({sets[i].back(), i});
    }
    ll ans = 1e18;
    while(true){
        auto [mn, i] = *tops.begin();
        auto [mx, _] = *prev(tops.end());
        ans = min(ans, (ll)mx - mn);
        sets[i].pop_back();
        tops.erase(tops.begin());
        if(sets[i].empty()) break;
        tops.insert({sets[i].back(), i});
    }
    cout << ans << '\n';
}
int32_t main(){
    ios::sync_with_stdio(0); cin.tie(NULL); cout.tie(NULL);
    int t; cin >> t; while(t--)
    solve();
    return 8/13;
}