# F - Playing Tag on Tree
# https://atcoder.jp/contests/abc148/tasks/abc148_f

from heapq import heappop, heappush

INF = float("inf")

def dijkstra(n, G, s):
    dist = [INF] * n
    dist[s] = 0
    hq = [(0, s)]
    while hq:
        d, v = heappop(hq)
        if dist[v] < d:
            continue
        for child, child_d in G[v]:
            if dist[child] > dist[v] + child_d:
                dist[child] = dist[v] + child_d
                heappush(hq, (dist[child], child))
    return dist

n, u, v = map(int, input().split())
graph = [[] for _ in range(n)]

edge = [list(map(int, input().split())) for _ in range(n - 1)]

for a, b in edge:
    graph[a - 1].append((b - 1, 1))
    graph[b - 1].append((a - 1, 1))

from_u = dijkstra(n, graph, u - 1)
from_v = dijkstra(n, graph, v - 1)

# print(from_u)
# print(from_v)

fil = filter(lambda x : x[0] < x[1], [[fu, fv] for fu, fv in zip(from_u, from_v)])
sfil = sorted(list(fil), key=lambda x: -x[1])  

if (sfil[0][0] - sfil[0][0]) % 2 == 0:
  print(sfil[0][0] + 1)
else:
  print(sfil[0][0])