#include <cstdio>
#include <algorithm>
#include <vector>
#include <cstring>
#include <numeric>
#include <cassert>
#include <cmath>
#include <map>
#include <set>

using namespace std;
typedef unsigned int uint;
typedef long long LL;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<VI> VVI;
typedef vector<VVI> VVVI;
typedef vector<VL> VVL;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef vector<PII > VPII;
typedef vector<VPII> VVPII;

#define FOR(k,a,b) for(int k(a); k < (b); ++k)
#define REP(k,a) for(int k=0; k < (a); ++k)
#define ABS(a) ((a)>0?(a):-(a))

vector<int> p;//parent
vector<int> si;//component size

int get(int v)
{
	int u = v;
	while (p[u] != u)
	{
		u = p[u];
	}
	while (p[v] != v)
	{
		int tmp = p[v];
		p[v] = u;
		v = tmp;
	}
	return u;
}

void combine(int u, int v)
{
	int a = get(u);
	int b = get(v);
	p[a] = b;
	si[b] += si[a] + 1;
}

LL calc(int z)
{
	return LL(z)*(z + 1) / 2;
}

int main(int argc, char** argv) {
#ifdef HOME
  	freopen("in.txt", "rb", stdin);
  	freopen("out.txt", "wb", stdout);
#endif
	int N;
	scanf("%d", &N);
	vector<pair<int, pair<int,int> > > edges(N-1);
	vector<int> weight(N-1);
	p.resize(N);
	si.resize(N);
	LL sumsave = 0, sum = 0;
	REP(i, N) p[i] = i;
	REP(i, N-1)
	{
		scanf("%d %d %d", &edges[i].second.first, &edges[i].second.second, &edges[i].first);
		sum += edges[i].first;
		--edges[i].second.first, --edges[i].second.second;
	}
	sort(edges.begin(), edges.end());
	
	REP(i, edges.size())
	{
		int u = get(edges[i].second.first);
		int v = get(edges[i].second.second);
		int weight = edges[i].first;
		LL ss = si[u] + si[v] + 1;
		sumsave += (calc(ss)-calc(si[u])-calc(si[v])) * weight;
		combine(u, v);
	}

	long double res = sumsave;
	res /= calc(N-1);
	res = sum - res;
	printf("%Lf\n",res);
	return 0;
}