from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
import itertools
from itertools import permutations,combinations
import sys
import bisect
import string
import math
import time
import random
def I():
    return int(input())
def MI():
    return map(int,input().split())
def LI():
    return [int(i) for i in input().split()]
def LI_():
    return [int(i)-1 for i in input().split()]
def StoI():
    return [ord(i)-97 for i in input()]
def ItoS(nn):
    return chr(nn+97)
def GI(V,E,Directed=False,index=0):
    org_inp=[]
    g=[[] for i in range(n)]
    for i in range(E):
        inp=LI()
        org_inp.append(inp)
        if index==0:
            inp[0]-=1
            inp[1]-=1
        if len(inp)==2:
            a,b=inp
            g[a].append(b)
            if not Directed:
                g[b].append(a)
        elif len(inp)==3:
            a,b,c=inp
            aa=(inp[0],inp[2])
            bb=(inp[1],inp[2])
            g[a].append(bb)
            if not Directed:
                g[b].append(aa)
    return g,org_inp
def show(*inp,end='\n'):
    if show_flg:
        print(*inp,end=end)
YN=['Yes','No']
mo=10**9+7
inf=float('inf')
l_alp=string.ascii_lowercase
u_alp=string.ascii_uppercase
ts=time.time()
#sys.setrecursionlimit(10**5)
input=lambda: sys.stdin.readline().rstrip()

def ran_input():
    import random
    n=random.randint(4,16)
    rmin,rmax=1,10
    a=[random.randint(rmin,rmax) for _ in range(n)]
    return n,a

show_flg=False
show_flg=True
#for i in range(n):

ans=0

n,t,a=LI_()
n+=1
g,c=GI(n,n-1)

#for i in range(1,30):


#vにはdepthをセット & -1 で未訪問フラグ

v=[-1]*(n)

def bfs(x):  # x スタート
    p=deque()
    p.append(x)
    v[x]=0
    while p:
        c=p.popleft()
        for i in g[c]:
            if v[i]==-1:
                p.append(i)
                v[i]=v[c]+1
    return

w=[-1]*(n)

def t_bfs(x):  # x スタート
    ans=0
    p=deque()
    p.append(x)
    w[x]=0
    while p:
        c=p.popleft()
        for i in g[c]:
            if w[i]==-1:
                p.append(i)
                #w[i]=w[c]+1
                if w[c]+1==v[i]:
                    ans=max(ans,v[i])
                if w[c]+1>=v[i]:
                #if w[c]+1>v[i]:
                    w[i]=-inf
                else:
                    w[i]=w[c]+1
                    ans=max(ans,w[i])
                    
    return ans


bfs(a)
ans=t_bfs(t)

#print(max(0,ans-1))
print(ans)
