pagehopping

import sys
import os
if not os.environ.get("ONLINE_JUDGE"):
    sys.stdin = open('./in.txt', 'r')
    sys.stdout = open('./out.txt', 'w')

inf = sys.maxsize
test = 1
while True:
    values = list(map(int,input().split()))
    values = values[:-2]
    dist = [[inf for _ in range(101)] for _ in range(101)]

    for i in range(101):
        dist[i][i] = 0

    if values.count(0) == len(values):
        break

    # print(values)
    vertex = []
    i,j = 0,1
    while j < len(values):
        u = values[i]
        v = values[j]
        vertex.append(u)
        vertex.append(v)
        dist[u][v] = 1
        i += 2
        j += 2

    vertex = set(vertex)

    for k in range(101):
        for i in range(101):
            for j in range(101):
                if dist[i][j] > dist[i][k] + dist[k][j]:
                    dist[i][j] = dist[i][k]+dist[k][j]

    sum = 0
    for i in range(101):
        for j in range(101):
            if dist[i][j] != inf:
                sum += dist[i][j]

    # print(sum)
    avg = sum / (len(vertex)* (len(vertex)-1))
    # print(avg)

    print(f"Case {test}: average length between pages = ",end="")
    print("{0:.3f}".format(avg))
    test += 1