#include <bits/stdc++.h>
using namespace std;
//#define int long long
//TEMPLATE START---------------8<---------------8<---------------8<---------------8<---------------//
typedef long long ll;       typedef long double ld;  typedef pair<int,int> pii; typedef pair<ll,ll> pll;  typedef vector<int> vi;   typedef vector<ll> vl;
typedef vector<string> vst; typedef vector<bool> vb; typedef vector<ld> vld;    typedef vector<pii> vpii; typedef vector<pll> vpll; typedef vector<vector<int> > vvi;
const int INF = (0x7FFFFFFFL); const ll INFF = (0x7FFFFFFFFFFFFFFFL); const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const int MOD = 1e9 + 7;       const int MODD = 998244353;            const string alphabet = "abcdefghijklmnopqrstuvwxyz";
const double PI = acos(-1.0);  const double EPS = 1e-9;               const string Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
int dx[9] = { 1, 0, -1,  0,  1, -1, -1, 1, 0 };
int dy[9] = { 0, 1,  0, -1, -1, -1,  1, 1, 0 };
#define ln '\n'
#define scnaf scanf
#define sacnf scanf
#define sancf scanf
#define SS(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t){cin >> t;}template<typename First, typename...Rest> void MACRO_VAR_Scan(First& first, Rest&...rest){cin >> first;MACRO_VAR_Scan(rest...);}
#define SV(type,c,n) vector<type> c(n);for(auto& i:c)cin >> i;
#define SVV(type,c,n,m) vector<vector<type>> c(n,vector<type>(m));for(auto& r:c)for(auto& i:r)cin >> i;
template<class T,class U>ostream &operator<<(ostream &o,const pair<T,U>&j){o<<"{"<<j.first<<", "<<j.second<<"}";return o;}
template<class T,class U>ostream &operator<<(ostream &o,const map<T,U>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const set<T>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const multiset<T>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const vector<T>&j){o<<"{";for(int i=0;i<(int)j.size();++i)o<<(i>0?", ":"")<<j[i];o<<"}";return o;}
inline int print(void){cout << endl; return 0;}
template<class Head> int print(Head&& head){cout << head;print();return 0;} template<class Head,class... Tail> int print(Head&& head,Tail&&... tail){cout<<head<<" ";print(forward<Tail>(tail)...);return 0;}
#ifdef LOCAL
inline int dump(void){cerr << endl; return 0;}
template<class Head> int dump(Head&& head){cerr << head;dump();return 0;}
template<class Head,class... Tail> int dump(Head&& head,Tail&&... tail){cerr<<head<<" ";dump(forward<Tail>(tail)...);return 0;}
#define debug(...) do{cerr<<__LINE__<<":	"<<#__VA_ARGS__<<" = ";dump(__VA_ARGS__);}while(0)
#define ER(x)  cerr << #x << " = " << (x) << endl;
#define ERV(v) {cerr << #v << " : ";for(const auto& xxx : v){cerr << xxx << " ";}cerr << "\n";}
#else
#define dump(...)
#define debug(...)
#define ER(x)
#define ERV(v)
#endif
template<typename T> void PA(T &a){int ASIZE=sizeof(a)/sizeof(a[0]);for(int ii=0;ii<ASIZE;++ii){cout<<a[ii]<<" \n"[ii==ASIZE-1];}}
template<typename T> void PV(T &v){int VSIZE=v.size();for(int ii=0;ii<VSIZE;++ii){cout<<v[ii]<<" \n"[ii==VSIZE-1];}}
inline int YES(bool x){cout<<((x)?"YES":"NO")<<endl;return 0;} inline int Yes(bool x){cout<<((x)?"Yes":"No")<<endl;return 0;}  inline int yes(bool x){cout<<((x)?"yes":"no")<<endl;return 0;}
inline int yES(bool x){cout<<((x)?"yES":"nO")<<endl;return 0;} inline int Yay(bool x){cout<<((x)?"Yay!":":(")<<endl;return 0;}
template<typename A,typename B> void sankou(bool x,A a,B b){cout<<((x)?(a):(b))<<endl;}
#define _overload3(_1,_2,_3,name,...) name
#define _REP(i,n) REPI(i,0,n)
#define REPI(i,a,b) for(ll i=ll(a);i<ll(b);++i)
#define REP(...) _overload3(__VA_ARGS__,REPI,_REP,)(__VA_ARGS__)
#define _RREP(i,n) RREPI(i,n,0)
#define RREPI(i,a,b) for(ll i=ll(a);i>=ll(b);--i)
#define RREP(...) _overload3(__VA_ARGS__,RREPI,_RREP,)(__VA_ARGS__)
#define EACH(e,v) for(auto& e : v)
#define PERM(v) sort((v).begin(),(v).end());for(bool c##p=1;c##p;c##p=next_permutation((v).begin(),(v).end()))
#define ADD(a,b) a=(a+ll(b))%MOD
#define MUL(a,b) a=(a*ll(b))%MOD
inline ll MOP(ll x,ll n,ll m=MOD){ll r=1;while(n>0){if(n&1)(r*=x)%=m;(x*=x)%=m;n>>=1;}return r;}
inline ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}inline ll lcm(ll a,ll b){return a*b/gcd(a,b);}inline ll POW(ll a,ll b){ll c=1ll;do{if(b&1)c*=1ll*a;a*=1ll*a;}while(b>>=1);return c;}
template<typename T,typename A,typename B> inline bool between(T x,A a,B b) {return ((a<=x)&&(x<b));}template<class T> inline T sqr(T x){return x*x;}
template<typename A,typename B> inline bool chmax(A &a,const B &b){if(a<b){a=b;return 1;}return 0;}
template<typename A,typename B> inline bool chmin(A &a,const B &b){if(a>b){a=b;return 1;}return 0;}
#define tmax(x,y,z) max((x),max((y),(z)))
#define tmin(x,y,z) min((x),min((y),(z)))
#define PB emplace_back
#define MP make_pair
#define MT make_tuple
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define SORT(v) sort((v).begin(),(v).end())
#define RSORT(v) sort((v).rbegin(),(v).rend())
#define EXIST(s,e) (find((s).begin(),(s).end(),(e))!=(s).end())
#define EXISTST(s,c) (((s).find(c))!=string::npos)
#define POSL(x,val) (lower_bound(x.begin(),x.end(),val)-x.begin())
#define POSU(x,val) (upper_bound(x.begin(),x.end(),val)-x.begin())
#define GEQ(x,val) (int)(x).size() - POSL((x),(val))
#define GREATER(x,val) (int)(x).size() - POSU((x),(val))
#define LEQ(x,val) POSU((x),(val))
#define LESS(x,val) POSL((x),(val))
#define SZV(a) int((a).size())
#define SZA(a) sizeof(a)/sizeof(a[0])
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
#define MEMINF(a) memset(a,0x3f,sizeof(a))
#define FILL(a,b) memset(a,b,sizeof(a))
#define UNIQUE(v) sort((v).begin(),(v).end());(v).erase(unique((v).begin(),(v).end()),(v).end())
struct abracadabra{
    abracadabra(){
        cin.tie(0); ios::sync_with_stdio(0);
        cout << fixed << setprecision(20);
        cerr << fixed << setprecision(5);
    };
} ABRACADABRA;

//TEMPLATE END---------------8<---------------8<---------------8<---------------8<---------------//

struct HLD {
    static const int MAXLOG = 20;
    int V, E = 0;
    vector<int> dep, sz, pathidx, child, idx;
    vector<vector<int>> edge, paths, dbl;
    HLD(int V) : V(V) {
        edge.resize(V);
        paths.resize(V);
        dep = sz = pathidx = child = idx = vector<int>(V, 0);
        dbl.resize(MAXLOG + 1, vector<int>(V, 0));
    }
    void add_edge(int a, int b) {
        edge[a].emplace_back(b);
        edge[b].emplace_back(a);
        ++E;
    }
    void dfs(int cur, int prv) {
        dbl[0][cur] = prv;
        sz[cur] = 1;
        child[cur] = idx[cur] = -1;
        pathidx[cur] = cur;
        dep[cur] = (prv == -1) ? 0 : (dep[prv] + 1);
        for (int i = 0; i < edge[cur].size(); ++i) if (edge[cur][i] != prv) {
            int nxt = edge[cur][i];
            dfs(nxt, cur);
            sz[cur] += sz[nxt];
            if (child[cur] == -1 or sz[nxt] > sz[child[cur]]) {
                child[cur] = nxt;
                idx[cur] = i;
            }
        }
    }
    void dfs2(int cur, int prv) {
        if (prv != -1 and child[prv] == cur) pathidx[cur] = pathidx[prv];
        paths[pathidx[cur]].emplace_back(cur);
        for (auto& nxt: edge[cur]) if (nxt != prv) dfs2(nxt, cur);
    }
    int lca(int u, int v) {
        if (dep[u] > dep[v]) swap(u, v);
        for (int i = MAXLOG - 1; i >= 0; --i) if (dep[v] - dep[u] >= 1 << i) v = dbl[i][v];
        for (int i = MAXLOG - 1; i >= 0; --i) if (dbl[i][u] != dbl[i][v]) {
            u = dbl[i][u];
            v = dbl[i][v];
        }
        return (u == v) ? u : dbl[0][u];
    }
    void build(int root = 0) {
        dfs(root, root);
        dfs2(root, root);
        for (int i = 0; i < MAXLOG; ++i) {
            for (int j = 0; j < V; ++j) {
                dbl[i + 1][j] = dbl[i][dbl[i][j]];
            }
        }
    }
};

/*
    > Heavy-Light Decomposition
        * 木上のクエリをセグ木で処理できるようにするやつ
        * 基本的に頂点にコストがあるタイプのクエリを処理できる
        * 辺上にコストがあるときは, depが大きい方に大して処理するとよい
    $ Variables
        *     dep : 頂点の深さ
        *      sz : 頂点の部分木の大きさ
        * pathidx : 頂点が属するpathのidx (始点の頂点番号)
        *   child : 頂点の子のidx (なければ-1)
        *     idx : HeavyEdgeを繋げるためのやつ
        *    edge : グラフ
        *   paths : 各pathに属する点のvector
        *     dbl : LCA用のダブリングテーブル
    $ Functions
        @ 前処理
            HLD hld(100100);        // 100100頂点の木
            hld.add_edge(2, 6);     // 頂点2, 6間の無向辺を追加
            hld.build();            // いろいろbuildする
        @ クエリ
            hld.lca(a, b);          // 頂点a, bのLCA
            SegmentTree seg[100100];
            for (int i = 0; i < V; ++i)
                if (hld.pathidx[i] == i)
                    seg[i] = SegmentTree(hld.paths[i].size() + 2, init, func);
            query(u, v);            // 頂点u, v間のクエリ
            seg[hld.pathidx[u]].replace(hld.dep[u] - hld.dep[hld.pathidx[u]], val);
                                    // 頂点uに対するコスト変更クエリ
            遅延セグメント木のときはqueryみたいにすればいい
*/

const int MAXN = 100100;
HLD hld(MAXN);

vector<vector<int>> graph;

map<pair<int, int>, int> lll;
int dist(int a, int b) {
    if (lll.count(make_pair(a, b))) return lll[make_pair(a, b)];
    int lca = hld.lca(a, b);
    return lll[make_pair(a, b)] = hld.dep[a] + hld.dep[b] - 2 * hld.dep[lca];
}

set<int> leaves;
int far_leaf(int tak, int aok) {
    int mx = 0;
    EACH(leaf, leaves) {
        if (dist(leaf, tak) >= dist(leaf, aok)) continue;
        chmax(mx, dist(leaf, aok) - 1);
        debug(leaf);
    }   
    return mx;
}

signed main() {

    SS(int, N, U, V);
    --U, --V;

    map<int, int> cnt;

    graph.resize(N);
    REP(i, N - 1) {
        SS(int, A, B);
        --A, --B;
        cnt[A]++;
        cnt[B]++;
        graph[A].emplace_back(B);
        graph[B].emplace_back(A);
        hld.add_edge(A, B);
    }
    hld.build();

    EACH(e, cnt) if (e.second == 1) leaves.emplace(e.first);

    print(far_leaf(U, V));

}


Measured memory_bytes: 42232