n,g,s = map(int,input().split())
G = [[] for _ in range(n)]
for i in range(n):
  a,b = map(int,input().split())
  G[a-1].append(b-1)
  G[b-1].append(a-1)
  
def dfs(V,v,d):
  if V[v] != -1:
    return
  V[v] = d
  for nv in G[v]:
    dfs(V,nv,d+1)
  
if len(G[g-1])==1 and G[g-1][0]==s-1:
  print(0)
else:
  vs = [-1]*n
  vg = [-1]*n
  dfs(vs,s-1,0)
  dfs(vg,g-1,0)
  
  ans = 0
  for i in range(n):
    if vs[i]>vg[i]:
      ans = max(ans,vs[i])
      
  print(ans-1)