from collections import deque

def BFS(t):
    tree=[[] for i in range(n)]
    for i in list:
      tree[i[0]-1].append(i[1]-1)
      tree[i[1]-1].append(i[0]-1)

    dist=[-1 for i in range(n)]
    dist[t-1]=0

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

n,u,v=map(int,input().split())
list=[list(map(int,input().split())) for i in range(n-1)]

u_dist=BFS(u)
v_dist=BFS(v)

l=0
for u_d,v_d in zip(u_dist,v_dist):
 if u_d<v_d:
  l=max(l,v_d)

print(l-1)