n,u,v=map(int, input().split())
u-=1
v-=1
 
graph = [[] for _ in range(n+1)]
for i in range(n-1):
    a, b=map(int, input().split())
    graph[a-1].append(b-1)
    graph[b-1].append(a-1)
 

dist=[[-1, -1] for _ in range(n)]
dist[v][0]=0
dist[u][1]=0
 
def dfs(graph,v,k):
    ver=graph[v]
    for vers in ver:
        if dist[vers][k]==-1:
            dist[vers][k]=dist[v][k]+1
            dfs(graph,vers,k)
 
 
dfs(graph,v,0)
dfs(graph,u,1)
dist.sort(reverse=True)
 
for i in range(10**6):
    if dist[i][0]-dist[i][1]>=1:
        print(dist[i][0]-1)
        break