#include <bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define REP(a,b) for(int a=0;a<(b);++a)
#define REP1(i,n) for(int i=1;i<=(n);++i)
#define debug(x) cerr<<#x<<": "<<x<<'\n'
#define all(x) (x).begin(),(x).end()
#define YES() printf("YES\n")
#define NO() printf("NO\n")
#define isYES(x) printf("%s\n",(x) ? "YES" : "NO")
#define Yes() printf("Yes\n")
#define No() printf("No\n")
#define isYes(x) printf("%s\n",(x) ? "Yes" : "No")
#define isPossible(x) printf("%s\n",(x) ? "Possible" : "Impossible")
#define SZ(x) ((int)(x).size())
#define pb push_back
#define mp make_pair
// #define INF (1<<29)

const long long INF = 1LL<<60;

#define Sp(p) cout<<setprecision(25)<< fixed<<p<<endl
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
#define pie 3.14159265358979323846
using namespace std;
typedef pair<int, int> P;
typedef pair<ll, ll> LP;
typedef pair<int, P> PP;
typedef pair<ll, LP> LPP;
template<class T=int>
T in(){T x;cin>>x;return (x);}
template<class T>
void print(T& x){cout<<x<<'\n';}

const int MOD =(int)1e9+7;
// const int mod =(int)998244353;
const int mod =(int)1e9+7;

const int MAX =510000;
ll fac[MAX],finv[MAX],inv[MAX];
void COMint(){
    fac[0]=fac[1]=1;
    finv[0]=finv[1]=1;
    inv[1]=1;
    for(int i=2;i<MAX;i++){
        fac[i]=fac[i-1]*i%MOD;
        inv[i]=MOD-inv[MOD%i]*(MOD/i)%MOD;
        finv[i]=finv[i-1]*inv[i]%MOD;
    }
}
ll COM(int n,int k){
    if(n<k) return 0;
    if(n<0||k<0)return 0;
    return fac[n]*(finv[k]*finv[n-k]%MOD)%MOD;
}
ll gcd(ll a,ll b){
    if(a<0)a=-a;
    if(b<0)b=-b;
    if(b==0)return a;
    if(a>b){
        swap(a,b);
    }
    return gcd(a,b%a);
}
ll lcm(ll a,ll b){
  if(a<0)a=-a;
  if(b<0)b=-b;
    ll g;g=gcd(a,b);
    return b/g*a;
}
bool compare_by_b(pair<int, int> a, pair<int, int> b) {
    if(a.second != b.second){
        return a.second < b.second;
    }else{
        return a.first < b.first;
    }
}
bool compare_by_a(pair<int, int> a, pair<int, int> b) {
    if(a.first != b.first){
        return a.first < b.first;
    }else{
        return a.second < b.second;
    }
}
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
ll RS(ll N,ll P,ll m){
    if(P==0){
        return 1;
    }else{
        if(P%2==0){
            ll t=RS(N,P/2,m);
            return t*t%m;
        }else{
            return N*RS(N,P-1,m)%m;
        }
    }
}
bool greater_pair(pair<ll,ll> a,pair<ll,ll> b){
    if(a.first!=b.first){
        return a.first>b.first;
    }else{
        return a.second>b.second;
    }
}
int N;
vector<vector<int>> to(100100);
vector<int> parent(100100,-1);
vector<int> depth(100100,-1);
vector<int> level(100100,-1);
vector<int>checked(100100,0);

int f(int i){
    checked[i]=1;
    int res=0;
    REP(ni,to[i].size()){
        if(checked[to[i][ni]]!=0)continue;
        parent[to[i][ni]]=i;
        level[to[i][ni]]=level[i]+1;
        res=max(res,1+f(to[i][ni]));
    }
    return depth[i]=res;
}
int main() {
    ios::sync_with_stdio(false);
    cin>>N;
    int v,u;cin>>u>>v;--v,--u;
    REP(i,N-1){
        int a,b;cin>>a>>b;--a;--b;
        to[a].pb(b);
        to[b].pb(a);
    }
    vector<int> taka(100100,-1);
    vector<int> ao(100100,-1);
    queue<int> q;
    q.push(u);
    taka[u]=0;
    while(!q.empty()){
        int i=q.front();q.pop();
        REP(ni,to[i].size()){
            if(taka[to[i][ni]]>=0)continue;
            taka[to[i][ni]]=taka[i]+1;
            q.push(to[i][ni]);
        }
    }
    ao[v]=0;
    q.push(v);
    while(!q.empty()){
        int i=q.front();q.pop();
        REP(ni,to[i].size()){
            if(ao[to[i][ni]]>=0)continue;
            ao[to[i][ni]]=ao[i]+1;
            q.push(to[i][ni]);
        }
    }
    int ans=0;
    REP(i,N){
        if(taka[i]<ao[i]){
            ans=max(ans,ao[i]-1);
        }
    }
    cout<<ans<<endl;
    return 0;
} 


Measured memory_bytes: 11308