nii=lambda:map(int,input().split())
from collections import deque

n,u,v=nii()
u-=1
v-=1

tree=[[] for i in range(n)]
for i in range(n-1):
  a,b=nii()
  a-=1
  b-=1
  tree[a].append(b)
  tree[b].append(a)

def BFS(s):
  dist=[-1 for i in range(n)]
  dist[s]=0

  que=deque()
  que.append(s)

  while que:
    x=que.popleft()
    for i in tree[x]:
      if dist[i]==-1:
        que.append(i)
        dist[i]=dist[x]+1
  return dist

dist_t=BFS(u)
dist_a=BFS(v)

ans=0
for i in range(n):
  if dist_a[i]>dist_t[i]:
    ans=max(ans,dist_a[i]-1)

print(ans)