n = int(input())
edges = []
for i in range(n-1):
    a, b, c = map(int, input().strip().split())
    edges.append((c, a-1, b-1))
edges.sort()

parent = [-1]*n
def find(n):
    if parent[n] < 0:
        return n
    else:
        pt = find(parent[n])
        parent[n] = pt
        return pt

total = 0
num = 0
den = 0
for c, a, b in edges:
    a = find(a)
    b = find(b)
    assert a != b
    total += c
    num += parent[a] * parent[b] * 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

print("%.11f" % (total - num / den))