#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vint;
typedef vector<double> vd;
typedef vector<ll> vll;
typedef vector<string> vstr;
typedef vector<vector<int>> vvint;
typedef vector<pair<int, int>> vpii;
typedef vector<pair<ll, ll>> vpll;
typedef priority_queue<int, vector<int>, greater<int>> spqint; //小さい順に取り出し
typedef priority_queue<ll, vector<ll>, greater<ll>> spqll;     //小さい順に取り出し
typedef priority_queue<int, vector<int>, less<int>> bpqint;    //大きい順に取り出し
typedef priority_queue<ll, vector<ll>, less<ll>> bpqll;        //大きい順に取り出し

#define REP(i, n) for (ll i = 0; i < (ll)(n); i++)
#define FOR(i, a, b) for (ll i = (ll)a; i < (ll)b; i++)
#define IREP(i, v) for (auto i = (v).begin(); i != (v).end(); i++)
#define FI first
#define SE second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define PF push_front
#define TS to_string
#define BS binary_search
#define LB lower_bound
#define UB upper_bound
#define NP next_permutation
#define ALL(v) (v).begin(), (v).end()
#define SZ(x) (ll) x.size()
#define SP(x) setprecision((ll)x)


int INF = 1e9;
int NIL = -1;
ll MOD = 1000000007;
ll LINF = 1e18;
double EPS = 1e-9;
double PI = M_PI;
int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; } //最大公約数
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }     //最小公倍数

void yes(){ cout << "Yes" << endl;}
void no(){ cout << "No" << endl;}
//-----------------------------------------

//-----------------------------------------
int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    std::chrono::system_clock::time_point start,end;
    start = std::chrono::system_clock::now();

    ll n, u, v ;
    cin >> n >> u >> v ;
    vector<vector<ll>> graph(n,vector<ll>()) ;
    REP(i,n-1){
        ll a, b ;
        cin >> a >> b ;
        graph[a-1].PB(b-1) ;
        graph[b-1].PB(a-1) ;
    }
    vll distT(n,0) ;
    vll distA(n,0) ;

    vll seenT(n,0) ;
    vll seenA(n,0) ;

    queue<ll> que ;

    que.push(u-1) ;
    seenT[u-1] = 1 ;
    distT[u-1] = 0 ;

    while(!que.empty()){
        ll k = que.front() ;
        que.pop() ;
        REP(i,SZ(graph[k])){
            if(seenT[graph[k][i]]==0){
                que.push(graph[k][i]) ;
                seenT[graph[k][i]] = 1 ;
                distT[graph[k][i]] = distT[k] + 1 ;
            }
        }
    }

    que.push(v-1) ;
    seenA[v-1] = 1 ;
    distA[v-1] = 0 ;

    while(!que.empty()){
        ll k = que.front() ;
        que.pop() ;
        REP(i,SZ(graph[k])){
            if(seenA[graph[k][i]]==0){
                que.push(graph[k][i]) ;
                seenA[graph[k][i]] = 1 ;
                distA[graph[k][i]] = distA[k] + 1 ;
            }
        }
    }

    ll ans = 0 ;

    REP(i,n){
        //cout << i << " " << SZ(graph[i]) << " " <<distA[i] <<endl;
        if(SZ(graph[i])>=2 && distT[i]<=distA[i]){
            ans = max(ans,distA[i]) ;
        }
    }

    cout << ans <<endl;

    



    end = std::chrono::system_clock::now();
    auto elapsed = std::chrono::duration_cast< std::chrono::milliseconds >(end - start).count();
    //std::cout << elapsed <<"ms"<< std::endl;

    

    return 0;
}



Measured memory_bytes: 12716