#include<cstdio>
#include<vector>
#include<utility>
#include<algorithm>
using namespace std;

int main()
{
	int n;
	int ans = 2147483647;
	int left = 0, tot = 0;
	vector<pair<int, int>> v;
	vector<int> cnt;
	
	scanf("%d", &n);
	cnt.resize(n, 0);
	for (int i = 0; i < n; ++i)
	{
		int m;
		scanf("%d", &m);
		for (int j = 0; j < m; ++j)
		{
			int val;
			scanf("%d", &val);
			v.emplace_back(make_pair(val, i));
		}
	}
	sort(v.begin(), v.end());
	
	for (int i = 0; i < (int)v.size(); ++i)
	{
		if (++cnt[v[i].second] == 1)
		{
			++tot;
		}
		while (left < i && tot == n)
		{
			ans = min(ans, v[i].first - v[left].first);
			if (--cnt[v[left].second] == 0)
			{
				--tot;
			}
			++left;
		}
	}
	printf("%d\n", ans);
	return 0;
}