from collections import deque, defaultdict
from copy import deepcopy

N ,u, v = map(int,input().split())
graph = defaultdict(deque)
for _ in range(1,N):
    a, b = map(int,input().split())
    graph[a].append(b)
    graph[b].append(a)

depth = dict(zip(list(range(1,N+1)),[-1]*N))

def bfs(s):
    if graph[s]:
        for v in graph[s]:
            if depth[v] == -1:
                depth[v] = depth[s] + 1
                bfs(v)

depth = dict(zip(list(range(1,N+1)),[-1]*N))
depth[v] = 0
bfs(v)
depth_v = deepcopy(depth)

depth = dict(zip(list(range(1,N+1)),[-1]*N))
depth[u] = 0
bfs(u)
depth_u = depth
depth_v = sorted(depth_v.items(), key = lambda x:x[0])
depth_u = sorted(depth_u.items(), key= lambda x:x[0])
print(depth_v)
print(depth_u)
MAX = 0
for v,u in zip(depth_v, depth_u):
    print(v[1],u[1])
    if u[1] < v[1]:
        MAX = max(MAX,v[1])
print(MAX-1)