#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void read(T& x) {
  char c;
  bool f = 0;
  while ((c = getchar()) < '0' || '9' < c) f |= (c == '-');
  for (x = (c ^ 48); '0' <= (c = getchar()) && c <= '9';
       x = (x << 1) + (x << 3) + (c ^ 48))
    ;
  if (f) x = -x;
}
template <class T>
inline T read(const T sample) {
  T x = 0;
  char c;
  bool f = 0;
  while ((c = getchar()) < '0' || '9' < c) f |= (c == '-');
  for (x = (c ^ 48); '0' <= (c = getchar()) && c <= '9';
       x = (x << 1) + (x << 3) + (c ^ 48))
    ;
  return f ? -x : x;
}
template <class T>
void fwrit(const T x) {
  if (x < 0) return (void)(putchar('-'), fwrit(-x));
  if (x > 9) fwrit(x / 10);
  putchar(x % 10 ^ 48);
}
template <class T>
inline T Max(const T x, const T y) {
  return x > y ? x : y;
}
template <class T>
inline T Min(const T x, const T y) {
  return x < y ? x : y;
}
template <class T>
inline T fab(const T x) {
  return x > 0 ? x : -x;
}
inline long long gcd(const long long a, const long long b) {
  return b ? gcd(b, a % b) : a;
}
inline void getInv(long long inv[], const long long lim, const long long MOD) {
  inv[0] = inv[1] = 1;
  for (long long i = 2; i <= lim; ++i)
    inv[i] = 1ll * inv[MOD % i] * (MOD - MOD / i) % MOD;
}
inline long long mulMod(const long long a, const long long b,
                        const long long mod) {
  return ((a * b - (long long)((long double)a / mod * b + 1e-8) * mod) % mod +
          mod) %
         mod;
}
const long long MAXN = 1e5;
struct edge {
  long long to, nxt, w;
} e[MAXN * 2 + 5];
long long tail[MAXN + 5], ecnt;
inline void add_edge(const long long u, const long long v, const long long w) {
  e[++ecnt] = edge{v, tail[u], w};
  tail[u] = ecnt;
  e[++ecnt] = edge{u, tail[v], w};
  tail[v] = ecnt;
}
long long n;
inline void Init() {
  n = read(1);
  long long u, v, w;
  for (signed i = (1), i_end_ = (n - 1); i <= i_end_; ++i) {
    u = read(1), v = read(1), w = read(1);
    add_edge(u, v, w);
  }
}
typedef struct Node_Using_on_Sort {
  long long x, y, z;
  bool operator<(const Node_Using_on_Sort rhs) const {
    return y * rhs.x < rhs.y * x;
  }
} JZMorz;
long long sz[MAXN + 5], st[MAXN + 5], se[MAXN + 5];
JZMorz Q[MAXN + 5];
long long top;
inline void Dfs(const long long u, const long long fa) {
  sz[u] = 1;
  for (signed i = tail[u], v = e[i].to; i; i = e[i].nxt, v = e[i].to)
    if (v ^ fa) {
      Dfs(v, u);
      sz[u] += sz[v], st[u] += st[v];
    }
  top = 0;
  for (signed i = tail[u], v = e[i].to; i; i = e[i].nxt, v = e[i].to)
    if (v ^ fa) Q[++top] = JZMorz{sz[v], se[v] + e[i].w * 2, e[i].w};
  sort(Q + 1, Q + top + 1);
  for (signed i = (1), i_end_ = (top); i <= i_end_; ++i) {
    st[u] += 1ll * (se[u] + Q[i].z) * Q[i].x;
    se[u] += Q[i].y;
  }
}
signed main() {
  Init();
  Dfs(1, 0);
  printf("%.6f\n", (double)st[1] / (n - 1));
  return 0;
}
