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

class Dijkstra {
public:
    struct edge { long long v, dist; };

    struct state {
        long long v, cost;
        bool operator>(const state s) const { return cost > s.cost; }
    };

    const long long INF = (1LL << 60);
    long long N;
    vector< vector<edge> > E;

    Dijkstra(long long n): N(n), E(n) {}

    //有効グラフの時はこっち。u→vに距離dで結ぶ
    void add_directed_edge(long long u, long long v, long long d) {
        E[u].push_back((edge) { v, d });
    }

    //無向グラフの時はこっち。uとvを双方向に距離dで結ぶ
    void add_undirected_edge(long long u, long long v, long long d) {
        E[u].push_back((edge) { v, d });
        E[v].push_back((edge) { u, d });
    }

    //Sを始点として、他の頂点への最短経路を探す
    vector<long long> shortest_path(long long S) {
        vector<long long> dp(E.size(), INF);
        priority_queue<state, vector<state>, greater<state> > q;
        q.push((state) { S, 0 });
        while(!q.empty()) {
            long long v = q.top().v, cost = q.top().cost;
            q.pop();
            if(dp[v] <= cost) continue;
            dp[v] = cost;
            for(int i=0;i < E[v].size() ; i++) {
            long long nv = E[v][i].v, ncost = cost + E[v][i].dist;
            if(dp[nv] > ncost) q.push((state) { nv, ncost });
            }
        }
        return dp;
    }
};


signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(20);
    
    ll n,s,t;
    cin>>n>>s>>t;
    s--,t--;
    ll cnt[n]={};
    Dijkstra dij(n);
    for(int i=0;i<n-1;i++){
        int a,b;
        cin>>a>>b;
        a--,b--;
        cnt[a]++;
        cnt[b]++;
        dij.add_undirected_edge(a,b,1);
    }    
    vector<ll> ds=dij.shortest_path(s);
    vector<ll> dt=dij.shortest_path(t);
    ll ans = 0;
    if(cnt[s]==1){
        if(ds[t]==2){
            cout << 1 << endl;
            return 0;
        }
        else if(ds[t]==1){
            cout << 0 << endl;
            return 0;
        }
    }
    for(int i=0;i<n;i++){
        if(cnt[i]==1){
            if(ds[i] == dt[i]){
                ans = max(ans,dt[i]);
            }
            else {
                if(ds[t]%2==0) ans = max(ans,dt[i]);
                else ans = max(ans,dt[i]);
            }
        }
    }
    cout << ans << endl;

    
}