#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
#pragma GCC optimize("Ofast")
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout << (ans) << '\n'
#define STI(s) atoi(s.c_str())
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
#define itn int
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;} return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;} return 0;}
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);}};
const ll MOD = 1000000007;
const double EPS = 1e-9;
const int INF = INT_MAX;
const ll LLINF = 1LL<<60;

class DIJKSTRA {
public:
    int V;

    struct dk_edge {
        int to;
        int cost;
    };

    typedef pair<int, int> PI; //firstは最短距離、secondは頂点の番号
    vector<vector<dk_edge> >G;
    vector<int> d; //これ答え。d[i]:=V[i]までの最短距離
    vector<int> prev; //経路復元

    DIJKSTRA(int size) {
        V = size;
        G = vector<vector<dk_edge> >(V);
        prev = vector<int> (V, -1);
    }

    void add(int from, int to, int cost) {
        dk_edge e = {to, cost};
        G[from].push_back(e);
    }

    void dijkstra(int s) {
        //greater<P>を指定することでfirstが小さい順に取り出せるようにする
        priority_queue<PI, vector<PI>, greater<PI> > que;
        d = vector<int> (V, INF);
        d[s] = 0;
        que.push(PI(0, s));

        while(!que.empty()) {
            PI p = que.top();
            que.pop();
            int v = p.second;
            if(d[v] < p.first) continue;
            for(int i = 0; i < G[v].size(); i++) {
                dk_edge e = G[v][i];
                if(d[e.to] > d[v] + e.cost) {
                    d[e.to] = d[v] + e.cost;
                    prev[e.to] = v;
                    que.push(PI(d[e.to], e.to));
                }
            }
        }
    }
    vector<int> get_path(int t) {
        vector<int> path;
        for(;t != -1; t = prev[t]) {
            //tがsになるまでprev[t]をたどっていく
            path.push_back(t);
        }
        //このままだとt->sの順になっているので逆順にする
        reverse(path.begin(),path.end());
        return path;
    }
    void show() {
        for(int i = 0; i < d.size()-1; i ++) {
            cout << d[i] << " ";
        }
        cout << d[d.size()-1] << endl;
    }
};

int main(void){

    int n,a,b;

    scanf("%d %d %d",&n,&a,&b);
    a--;b--;
    int l,r;
    DIJKSTRA dk(n);
    rep(i,n-1){
        scanf("%d %d",&l,&r);
        l--;r--;
        dk.add(l,r,1);
        dk.add(r,l,1);
    }
    dk.dijkstra(a);
    vi v=dk.get_path(b);
    if(v.size()%2==0){
        r=v.size()/2;
        l=r-1;
    } else {
        int mid=v.size()/2;
        r=mid+1;
        l=mid-1;
    }
    int ans=0;
    ans+=(int)v.size()-r-1;
    bool tuen = 0;
    
    int efrom=v[l],eto=v[l+1];
    rep(i,dk.G[efrom].size()){
        if(dk.G[efrom][i].to==eto){
            dk.G[efrom].erase(dk.G[efrom].begin()+i);
            break;
        }
    }
    dk.dijkstra(efrom);

    int maxlen=0;
    rep(i,dk.d.size()){
        if(dk.d[i]!=INF)maxlen=max(maxlen,dk.d[i]);
    }
    fin(maxlen+ans+(r-l-1));


}


Measured memory_bytes: 7604