#include<bits/stdc++.h>
using namespace std;
#define int long long
#define PII pair<int,int>
#define lson 2*p
#define rson 2*p+1
#define x first
#define y second
//#define endl "\n"
constexpr int N=2e6+10;
constexpr int mod=998244353;
int n,m,cnt;
PII p[N];
void solve() {
    cin>>n;
    cnt=0;
    for(int i=1;i<=n;i++) {
        int k;
        cin>>k;
        for(int j=1;j<=k;j++) {
            int x;
            cin>>x;
            p[++cnt]={x,i};
        }
    }
    sort(p+1,p+cnt+1);
    int j=cnt;
    vector<int> num(n+10,0);
    int sum=1;
    num[p[cnt].y]++;
    int ans=1e18;
    for(int i=cnt;i>=1;i--) {
        while(j-1>=1&&sum<n) {
            j--;
            num[p[j].y]++;
            if(num[p[j].y]==1) sum++;
        }
        if(sum==n) {
            ans=min(ans,p[i].x-p[j].x);
            //cout<<i<<" "<<j<<" "<<p[i].x-p[j].x<<endl;
        }
        num[p[i].y]--;
        if(!num[p[i].y]) sum--;
    }
    cout<<ans<<endl;
}
signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int T=1;
    cin>>T;
    while(T--) {
        solve();
    }
}