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#include <bits/stdc++.h>
#define ll long long
#define il inline
#define ld long double
#define IOS ios::sync_with_stdio(false), cin.tie(0)
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define dep(i, a, b) for (int i = (a); i >= (b); i--)
using namespace std;
const int N = 100 + 5;
int n;
int k[105];
vector<int> a[105];
unordered_map<int, int> t[105];
signed main () {
	IOS;
	cin >> n;
	rep(i, 1, n) {
		cin >> k[i];
		rep(j, 1, k[i]) {
			int x;
			cin >> x;
			if (!t[i].count (x)) a[i].push_back(x);
			t[i][x]++;
		}
	}
	ld ans = 0;
	rep(i, 1, n) {
		rep(j, i + 1, n) {
			ld sum = 0;
			for (auto v : a[i]) {
				if (t[j].count (v))	sum += (ld)t[i][v] * (ld)t[j][v];
			}
			sum /= ((ld)k[i] * (ld)k[j]);
			ans = max(ans, sum);
		}
	}
	cout << fixed << setprecision(10) << ans << endl;
	return 0;
}