player = input()

# フェーズ 1
N = int(input())
k = N - 2
print(k)

# フェーズ 2
if player == "Alice":
    # Alice の操作
    tree = [set() for _ in range(N)]

    for _ in range(N - 1):
        u, v = [int(s) - 1 for s in input().split()]
        tree[u].add(v)
        tree[v].add(u)

    leaves = {i for i in range(N) if len(tree[i]) == 1}
    A = []

    for _ in range(k):
        v = min(leaves)
        leaves.discard(v)

        a = tree[v].pop()
        tree[a].discard(v)

        if len(tree[a]) == 1:
            leaves.add(a)

        A.append(a + 1)

    print(*A)

else:
    # Bob の操作
    A = [int(s) - 1 for s in input().split()]
    B = []

    edges = []

    for i, a in enumerate(A):
        b = min(set(range(N)) - set(A[i:] + B))

        B.append(b)
        edges.append((a, b))

    b = min(set(range(N)) - set(B))
    edges.append((b, N - 1))

    for u, v in edges:
        print(u + 1, v + 1)