import sys
sys.setrecursionlimit(100000000)
n,u,v=map(int,input().split())
visitedlist=[-1 for  i in range(n)]
kilist=[[] for i in range(n)]
anslist=[0 for i in range(n)]
searchedlist=[-1 for i in range(n)]
for i in range(n-1):
        a,b=map(int,input().split())
        kilist[a-1].append(b)
        kilist[b-1].append(a)
def search(P):
        searchedlist[P-1]=1
        visitedlist[P-1]=1
        flag=0
        for j in kilist[P-1]:
            if visitedlist[j-1]<0:
                visitedlist[j-1]=1
                flag+=1
                anslist[j-1]=anslist[P-1]+1
        else:
            pass

        if flag>0:    
            for k in kilist[P-1]:
                if searchedlist[k-1]==-1:
                    search(k)
search(v)
if v<u:
	print(anslist[u]-1)
else:
  if u>1:
  	print(v-u)
  else:
    print(v-2)
