#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <math.h>
#include <iomanip>
#include <limits>
#include <list>
#include <queue>
#include <tuple>
#include <map>
#include <stack>
#include <set>
using namespace std;
#define fast_io ios_base::sync_with_stdio (false) ; cin.tie(0) ; cout.tie(0) ;
#define MOD (long long int)(1e9+7)
#define ll long long int
#define rep(i,n) for(int i=0; i<(int)(n); i++)
#define reps(i,n) for(int i=1; i<=(int)(n); i++)
#define REP(i,n) for(int i=n-1; i>=0; i--)
#define REPS(i,n) for(int i=n; i>0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a)<(b)) ? (b) : (a))
#define chmin(a, b) a = (((a)>(b)) ? (b) : (a))
#define all(v) v.begin(), v.end()

const int N = (int)3e5;

ll mpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = mpow(a,b/2); return memo*memo%MOD;}
  else return mpow(a,b-1) * a % MOD;
}
ll lpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = lpow(a,b/2); return memo*memo;}
  else return lpow(a,b-1) * a;
}
ll gcd(ll a, ll b){
  if(b==0) return a;
  else return gcd(b, a%b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n){
  if(kaijo_memo.size() > n) return kaijo_memo[n];
  if(kaijo_memo.size() == 0) kaijo_memo.push_back(1);
  while(kaijo_memo.size() <= n) kaijo_memo.push_back(kaijo_memo[kaijo_memo.size()-1] * kaijo_memo.size() % MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n){
  if(gyaku_kaijo_memo.size() > n) return gyaku_kaijo_memo[n];
  if(gyaku_kaijo_memo.size() == 0) gyaku_kaijo_memo.push_back(1);
  while(gyaku_kaijo_memo.size() <= n) gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size()-1] * mpow(gyaku_kaijo_memo.size(), MOD-2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r){
  if(n == r) return 1;//0個の丸と-1個の棒みたいな時に時に効く？不安.
  if(n < r || r < 0) return 0;
  ll ret = 1;
  ret *= kaijo(n); ret %= MOD;
  ret *= gyaku_kaijo(r); ret %= MOD;
  ret *= gyaku_kaijo(n-r); ret %= MOD;
  return ret;
}

vector<int> G[(int)(1e5+3)];
bool oni[(int)(1e5+3)], nige[(int)(1e5+3)];
bool checkOni[(int)(1e5+3)], checkNige[(int)(1e5+3)];

int main(void){
  fast_io
  int u,v;
  int n;
  cin>>n>>u>>v;
  u--;v--;
  rep(i,n-1){
    int a,b;cin>>a>>b;a--;b--;
    G[a].push_back(b);
    G[b].push_back(a);
  }
  rep(i,n){
    checkOni[i] = false;
    checkNige[i] = false;
    oni[i] = false;
    nige[i] = false;
  }
  queue<int> A,B;
  A.push(u);B.push(v);
  oni[v] = true;
  checkOni[v] = true;
  checkNige[u] = true;
  ll countNige = 0;
  if(G[u].size() >= 2){
    countNige++;
    nige[u] = true;
  }
  ll ans = 0;
  while(true){
    queue<int> neA, neB;
    while(A.size() > 0){
      int a = A.front();A.pop();
      if(!nige[a] && a != u){
        continue;
      }
      rep(j,G[a].size()){
        int next = G[a][j];
        if(!checkNige[next] && !oni[next] && G[next].size() >= 2){
          checkNige[next] = true;
          nige[next] = true;
          countNige++;
          neA.push(next);
        }
      }
    }
    if(countNige == 0){
      break;
    }
    ans++;
    while(B.size() > 0){
      int b = B.front(); B.pop();
      rep(j,G[b].size()){
        int next = G[b][j];
        if(!checkOni[next]){
          checkOni[next] = true;
          oni[next] = true;
          if(nige[next]){
            nige[next] = false;
            countNige--;
          }
          neB.push(next);
        }
      }
    }
    A = neA;B = neB;
    if(countNige == 0){
      break;
    }
  }
  cout<<ans<<endl;
  return 0;
}

Measured memory_bytes: 6400