#include <bits/stdc++.h>
using namespace std;
#define N 100111
#define ll long long

typedef pair<int, pair<int,int> > edge;
#define c first
#define p second
#define a first
#define b second

edge edges[N];
ll parent[N];
int find(int n) {
    return parent[n] < 0 ? n : parent[n] = find(parent[n]);
}

void solve() {
    int n;
    scanf("%d", &n);
    ll num = 0, den = 0;
    for (int i = 0; i < n; i++) {
        parent[i] = -1;
    }
    double total = 0;
    for (int i = 1; i < n; i++) {
        int a, b;
        scanf("%d%d%d", &a, &b, &edges[i].c);
        edges[i].p.a = a-1;
        edges[i].p.b = b-1;
        total += edges[i].c;
    }
    sort(edges + 1, edges + n);
    for (int i = 1; i < n; i++) {
        int a = find(edges[i].p.a);
        int b = find(edges[i].p.b);
        num += parent[a]*parent[b]*edges[i].c;
        den += parent[a]*parent[b];
        if (parent[a] > parent[b]) {
            parent[b] += parent[a];
            parent[a] = b;
        } else {
            parent[a] += parent[b];
            parent[b] = a;
        }
    }
    printf("%.11f\n", total - (double) num / den);
}

int main() {
    solve();
}