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

def distances(sommet):
	d = [-1]*(N+1)
	File = [sommet]
	d[sommet] = 0
	while File:
		s = File.pop()
		for x in adj[s]:
			if d[x]==-1:
				d[x] = 1+d[s]
				File.append(x)
	return d


t = distances(u)
a = distances(v)

# ~ print(t)
# ~ print(a)

ans = -1
for i in range(1,N+1):
	if t[i]<a[i]:
		ans = max(ans, a[i]-1)
		
print(ans)
