#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <iostream>
#include <istream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#include <tuple>
#include <iomanip>
#include <climits>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
//typedef pair<ll, ll> P;
#define rep(i, n) for(ll i = 0; i < (n); i++)
#define revrep(i, n) for(ll i = (n)-1; i >= 0; i--)
#define pb push_back
#define f first
#define s second
#define chmin(x, y) x = min(x, y);
#define chmax(x, y) x = max(x, y);
/*
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll max3(ll a, ll b, ll c){return max(a, max(b, c));};
ll min3(ll a, ll b, ll c){return min(a, min(b, c));};
ll max4(ll a, ll b, ll c, ll d){return max(max(a, b), min(c, d));};
ll min4(ll a, ll b, ll c, ll d){return min(min(a, b), min(c, d));};
ll max5(ll a, ll b, ll c, ll d, ll e){return max(max(a, b), max3(c, d, e));};
ll min5(ll a, ll b, ll c, ll d, ll e){return min(min(a, b), min3(c, d, e));};
*/
//const ll INFL = LLONG_MAX;//10^18 = 2^60
const ll INFL = 1LL<<60;
//const int INF = INT_MAX;
const int INF = 1 << 30;//10^9
ll MOD = 1000000007;
//ll MOD = 998244353;

vector<ll> dy = {0, 1, 0, -1, 1, 1, -1, -1, 0};
vector<ll> dx = {1, 0, -1, 0, 1, -1, 1, -1, 0};

void pres(double A, ll x = 10){cout << fixed << setprecision(x) << A << endl;}
void BinarySay(ll x, ll y = 60){rep(i, y) cout << (x>>(y-1-i) & 1); cout << endl;}
ll cnt_bit(ll x){return __builtin_popcountll(x);}

ll pow_long(ll x, ll k){
  ll res = 1;
  while(k > 0){
    if(k % 2) res *= x;
    x *= x; k /= 2;
  }
  return res;
}

ll pow_mod(ll x, ll k){
  ll res = 1;
  while(k > 0){
    if(k % 2){
      res *= x; res %= MOD;
    }
    x *= x; x %= MOD;
    k /= 2;
  }
  return res;
}

ll inverse(ll x){return pow_mod(x, MOD - 2);};

ll gcd(ll a, ll b){
    if(b == 0) return a;
    return gcd(b, a % b);
}

ll lcm(ll x, ll y){return x / gcd(x, y) * y;};


//Lowest Common Ancestor
//initがあるの注意
const ll MAX_V = 200010;
const ll MAX_LOG_V = 30;
vector<ll> G[MAX_V];
ll root = 0;//根のノード番号

ll parent[MAX_LOG_V][MAX_V];
//2^k回親をたどって到達する点
//根を通り過ぎる時は-1

ll depth[MAX_V];
void dfs_lca(ll v, ll p, ll d){
  parent[0][v] = p;
  depth[v] = d;
  for(ll to : G[v]){
    if(to == p) continue;
    dfs_lca(to, v, d + 1);
  }
}

void init_lca(ll V = 200009){
  dfs_lca(root, -1, 0);
  for(ll k = 0; k + 1 < MAX_LOG_V; k++){
    for(ll v = 0; v < V; v++){
      if(parent[k][v] < 0) parent[k+1][v] = -1;
      else parent[k+1][v] = parent[k][parent[k][v]];
    }
  }
}

//uとvのLCAを求める
ll lca(ll u, ll v){
  if(depth[u] > depth[v]) swap(u, v);
  for(ll k = 0; k < MAX_LOG_V; k++){
    if((depth[v] - depth[u]) >> k & 1){
      v = parent[k][v];
    }
  }
  if(u == v) return u;
  //二分探索でLCAを求める
  for(ll k = MAX_LOG_V - 1; k >= 0; k--){
    if(parent[k][u] != parent[k][v]){
      u = parent[k][u];
      v = parent[k][v];
    }
  }
  return parent[0][u];
}

ll N;
ll u, v;

ll dfs(ll now){
  ll res = depth[now];
  for(ll to: G[now]){
    if(depth[to] < depth[now]) continue;
    res = max(res, dfs(to));
  }
  return res;
}

void solve(){
  cin >> N;
  cin >> u >> v;
  u--, v--;
  root = v;
  rep(i, N-1){
    ll a, b;
    cin >> a >> b;
    a--, b--;
    G[a].pb(b);
    G[b].pb(a);
  }
  root = v;
  init_lca();

  ll ans = 0;
  rep(i, N){
    if(depth[i] != depth[u]) continue;
    ll LCA = lca(i, u);
    if(depth[u] - depth[LCA] < depth[LCA]){
      ans = max(ans, dfs(i) - 1);
    }
  }
  cout << ans << endl;
}


int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}


Measured memory_bytes: 59776