#include<bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;
typedef vector<bool> vb;
typedef vector<vector<bool>> vvb;
typedef vector<vector<vector<bool>>> vvvb;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef vector<vpll> vvpll;
typedef vector<double> vd;
typedef vector<vd> vdd;

#define FOR(i,x,y) for(ll i=(ll)x; i<(ll)y; ++i)
#define REP(i,y) FOR(i, 0, y)
#define RFOR(i,x,y) for(ll i=(ll)x; i>=(ll)y; --i)
#define RREP(i,x) RFOR(i, x, 0)
#define ALL(a) a.begin(), a.end()
#define pb push_back
#define debug_print(x...) cerr << "line " << __LINE__ << " : "; debug_print_in(x);

template <typename First>
void debug_print_in(First first){
  cerr << first << endl;
  return;
}

template <typename First, typename... Rest>
void debug_print_in(First first, Rest... rest){
  cerr << first << " ";
  debug_print_in(rest...);
  return;
}

void IN(void){
  return;
}

template <typename First, typename... Rest>
void IN(First& first, Rest&... rest){
  cin >> first;
  IN(rest...);
  return;
}

template <typename First>
void OUT(First first){
  cout << first << endl;
  return;
}

template <typename First, typename... Rest>
void OUT(First first, Rest... rest){
  cout << first << " ";
  OUT(rest...);
  return;
}

template<class t, class u> void chmax(t&a,u b){if(a<b)a=b;};
template<class t, class u> void chmin(t&a,u b){if(a>b)a=b;};
int popcount(int t){return __builtin_popcount(t);} //GCC
int popcount(ll t){return __builtin_popcountll(t);} //GCC

template <typename T>
void vec_print(vector<T> VEC){
  REP(i, VEC.size()){
    cerr << VEC[i] << " ";
  }
  cerr << endl;
};

template <typename T>
void mat_print(vector<vector<T> > MAT){
  REP(i,MAT.size()){
    REP(j,MAT[i].size()){
      cerr << MAT[i][j] << " ";
    }
    cerr << endl;
  }
};

constexpr int INF = (1<<30);
constexpr ll INFLL = 1LL<<62;
constexpr long double EPS = 1e-12;
constexpr ll MOD = (ll)((1E+9)+7);

int main(){
  cin.tie(0); // cut the cin and cout (default, std::flush is performed after std::cin)
  ios::sync_with_stdio(false); // cut the iostream and stdio (DON'T endl; BUT "\n";)

  ll N, u, v;
  IN(N,u,v);
  u--; v--;
  vvll Tree(N);
  REP(i,N-1){
    ll A, B;
    IN(A,B);
    A--; B--;
    Tree[A].pb(B);
    Tree[B].pb(A);
  }


  vll w1(N,INFLL);
  stack<pair<ll, ll>> que1;
  que1.push(make_pair(u,0));
  while(que1.size()){
    pair<ll, ll> pos = que1.top(); que1.pop();
    if(w1[pos.first]<=pos.second) continue;
    w1[pos.first]=pos.second;
    REP(i,Tree[pos.first].size()){
      que1.push(make_pair(Tree[pos.first][i], pos.second+1));
    }
  }

  vll w2(N,INFLL);
  stack<pair<ll, ll>> que2;
  que2.push(make_pair(v,0));
  while(que2.size()){
    pair<ll, ll> pos = que2.top(); que2.pop();
    if(w2[pos.first]<=pos.second) continue;
    w2[pos.first]=pos.second;
    REP(i,Tree[pos.first].size()){
      que2.push(make_pair(Tree[pos.first][i], pos.second+1));
    }
  }

  ll ans = 0;
  REP(i,N){
    if(w2[i]<w1[i]) continue;
    if(Tree[i].size()==1) continue;
    if(w1[v]&1 && (w2[i]-w1[i])&1) ans = max(ans, w2[i]);
    if(w1[v]%2==0 && (w2[i]-w1[i])%2==0) ans = max(ans, w2[i]);
  }

  OUT(ans);

  return 0;
}

Measured memory_bytes: 8192