import sys
import numpy as np
from scipy.sparse.csgraph import dijkstra
from scipy.sparse import csr_matrix

read = sys.stdin.read
N, u, v, *AB = map(int, read().split())
AB = np.array(AB, np.int32).reshape(N - 1, 2)

a = AB[:, 0]
b = AB[:, 1]
d = np.ones_like(a, np.int32)

graph = csr_matrix((d, (a, b)), shape=(N + 1, N + 1))
takahashi, aoki = dijkstra(graph, directed=False, indices=[u, v])

print(int(aoki[takahashi < aoki].max()) - 1)