#include <cstdio>
#include <iostream>
#include <vector>

#pragma comment(linker, "/STACK:100000000")

#define N int(1e5)
#define pb push_back

using namespace std;

typedef long long LL;

vector<int> graph[N], cost[N];
bool was[N];
LL ans = 0;
int n;

int dfs(int v, int Cost)
{
    int res = 1;
    was[v] = true;
    for (int i = 0; i < graph[v].size(); i++)
    {
        int u = graph[v][i];
        if (!was[u])
        {
            res += dfs(u, cost[v][i]);
        }
    }
    LL add = Cost;
    add *= res * (n - res);
    ans += add;
    return res;
}

int main()
{
    cin >> n;
    for (int i = 1; i < n; i++)
    {
        int a, b, c;
        cin >> a >> b >> c;
        graph[a].pb(b);
        graph[b].pb(a);
        cost[a].pb(c);
        cost[b].pb(c);
    }

    dfs(1, 0);

    cout.precision(4);
    cout << fixed << double(ans) / ( LL(n) * (n - 1) / 2);

    return 0;
}