D

#include <iostream>
#include <iomanip>
#include <vector>
#include <array>
#include <set>

using namespace std;

#define int long long
#pragma comment(linker,"/STACK:1000000000,1000000000")

const long long INF = 1e18 + 7;
const int MAXN = 5e4 + 100;
const int N = 1e5 + 10;
const int MOD = 1e9 + 7;

vector<int> dp;
vector<int> used;
vector<vector<pair<int, int>>> g;

int n, sum = 0;

void dfs(int v) {
    used[v] = true;
    for (auto [to, w]: g[v]) {
        if (!used[to]) {
            dfs(to);
            dp[v] += dp[to];
            sum += w * dp[to] * (n - dp[to]);
        }
    }
    ++dp[v];
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    cin >> n;
    dp.resize(n, 0);
    used.resize(n, 0);
    g.resize(n);
    if (n == 1) {
        cout << 0 << '\n';
        return 0;
    }
    for (int i = 0; i < n - 1; ++i) {
        int u, v, w;
        cin >> u >> v >> w;
        --u, --v;
        g[u].emplace_back(v, w);
        g[v].emplace_back(u, w);
    }
    dfs(0);
    sum *= 2;
    cout << setprecision(10) << fixed;
    cout << double(sum) / double(n * (n - 1)) << '\n';
}