#include<bits/stdc++.h>
#define irep(i,l,r) for(int i = l; i <= r; ++i)
#define drep(i,r,l) for(int i = r; i >= l; --i)
#define ceil(pp,qq) (((pp)>0)^((qq)>0)?-Abs(pp)/Abs(qq):(pp)%(qq)?(pp)/(qq)+1:(pp)/(qq))
#define floor(pp,qq) (((pp)>0)^((qq)>0)?-ceil(abs(pp),abs(qq)):(pp)/(qq))
#define ll long long
using namespace std;
ll Abs(ll x){return x > 0 ? x : - x;}
inline ll read(){
	char ch = getchar();
	ll s = 0; bool w = 0;
	while(!isdigit(ch)){if(ch == '-')w = 1;ch = getchar();}
	while(isdigit(ch))s = (s << 3) + (s << 1) + (ch ^ 48), ch = getchar();
	return w ? - s : s;
}

const int itinf = 2e9;
const ll llinf = 4e18;
const int mod = 1000000007;
const int N = 500009;

void solve(){
	int n = read(), sum = 0, sz = 0;
	vector<array<int,2>> a;
	vector<int>cnt(n);
	irep(i,0,n-1){
		int c = read();
		irep(j,0,c-1){
			int x = read();
			a.push_back({x, i});
		}
	}
	sort(a.begin(), a.end());
	sz = a.size();
	int l = 0, r = -1;
	ll ans = itinf;
	while(l < sz){
		while(r + 1 < sz && sum < n){
			cnt[a[++ r][1]] ++;
			if(cnt[a[r][1]] == 1)++ sum;
		}
		if(sum == n)ans = min(ans, 1ll * a[r][0] - a[l][0]);
		cnt[a[l][1]] --;
		if(cnt[a[l ++][1]] == 0)-- sum;
	}
	printf("%lld\n", ans);
}

int main(){
	int T = read();
	while(T --){
		solve();
	}
	return 0;
}