/*
Author : N_o_o_B
Created : December 22 2019 17:43:49 
*/
#include <bits/stdc++.h>
using namespace std;
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
*/
#define TRACE
#ifdef TRACE
#define trace(...) { cerr<<"[ "; __trace__(#__VA_ARGS__, __VA_ARGS__);}
#undef endl
template <typename Arg1,typename Arg2>
ostream& operator << (ostream& out, const pair<Arg1,Arg2> &x) {
    return out<<"("<<x.first<<","<<x.second<<")";
}
 
template <typename Arg1>
ostream& operator << (ostream& out, const vector<Arg1> &a) {
    out<<"[";for(const auto &x:a)out<<x<<",";return out<<"]";
}
 
template <typename Arg1>
ostream& operator << (ostream& out, const set<Arg1> &a) {
    out<<"[";for(const auto &x:a)out<<x<<",";return out<<"]";
}
 
template <typename Arg1,typename Arg2>
ostream& operator << (ostream& out, const map<Arg1,Arg2> &a) {
    out<<"[";for(const auto &x:a)out<<x<<",";return out<<"]";
}
 
template <typename Arg1>
void __trace__(const string name, Arg1&& arg1){
    cerr << name << " : " << arg1 << " ] " << endl;
}
 
template <typename Arg1, typename... Args>
void __trace__(const string names, Arg1&& arg1, Args&&... args){
    const string name = names.substr(0,names.find(','));
    cerr<<name<<" : "<<arg1<<" | ";
    __trace__(names.substr(1+(int)name.size()), args...);
}
#else
    #define trace(args...)
#endif
 
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<pair<int,int>> vii;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<pair<ll,ll>> vll;
typedef vector<vl> vvl;
//typedef tree<pii, null_type, less<pii>, rb_tree_tag, tree_order_statistics_node_update> oset;
#define fori(i,n) for(int i=0;i<n;i++)
#define rep(i,a,b) for(int i=a;i<=b;i++)
#define repd(i,a,b) for(int i=a;i>=b;i--)
#define ford(i,n)  for(int i=n-1;i>=0;i--)
#define trav(x,a) for(auto& x:a)
#define all(x) x.begin(),x.end()
#define pb push_back
#define endl '\n'
#define sz(a) (int)a.size()
#define fi first
#define se second
clock_t time_p=clock();
void time_taken()
{
    time_p=clock()-time_p;
    cerr<<"Time Taken : "<<(float)(time_p)/CLOCKS_PER_SEC<<"\n";
}
//const ll mod=998244353;
const ll mod=1e9+7;
const ll INF=1e18;
const int maxn = 1e5+5;
vi adj[maxn];
vi d(maxn,-1);
int a;
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);
    cout.precision(12);cout << fixed;
    int n,b;
    cin >> n >> a >> b;
    fori(i,n-1){
        int u,v;
        cin >> u >> v;
        adj[u].pb(v);
        adj[v].pb(u);
    }

    queue<int> q;
    q.push(b);
    d[b] = 0;
    while(sz(q)){
        int u = q.front();
        q.pop();
        for(int v:adj[u]){
            if(d[v]==-1){
                d[v] = d[u] + 1;
                q.push(v);
            }
        }
    }
    vi d2(maxn,-1);
    q.push(a);
    int ans = 1;
    d2[a]=0;
    while(sz(q)){
        int u = q.front();
        if(d2[u] < d[u]){
            ans = max(ans,d[u]);
        }
        q.pop();
         for(int v:adj[u]){
            if(d2[v]==-1){
                d2[v] = d2[u] + 1;
                q.push(v);
            }
        }
    }
    cout << ans - 1 << endl;

    time_taken();
    return 0;
}

Measured memory_bytes: 6656