#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define MASK(i) (1LL << (i))
#define GETBIT(mask, i) (((mask) >> (i)) & 1)
#define ALL(v) (v).begin(), (v).end()
#define block_of_code if(true)
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll gcd(ll a, ll b){return __gcd(a, b);}
ll lcm(ll a, ll b){return a / gcd(a, b) * b;}
ll LASTBIT(ll mask){return (mask) & (-mask);}
int pop_cnt(ll mask){return __builtin_popcountll(mask);}
int ctz(ull mask){return __builtin_ctzll(mask);}
int logOf(ull mask){return 63 - __builtin_clzll(mask);}
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
ll rngesus(ll l, ll r){return l + (ull) rng() % (r - l + 1);}
double rngesus_d(double l, double r){
double cur = rngesus(0, MASK(60) - 1);
cur /= MASK(60) - 1;
return l + cur * (r - l);
}
template <class T1, class T2>
bool maximize(T1 &a, T2 b){
if (a < b) {a = b; return true;}
return false;
}
template <class T1, class T2>
bool minimize(T1 &a, T2 b){
if (a > b) {a = b; return true;}
return false;
}
template <class T>
void printArr(T container, string separator = " ", string finish = "\n", ostream &out = cout){
for(auto item: container) out << item << separator;
out << finish;
}
template <class T>
void remove_dup(vector<T> &a){
sort(ALL(a));
a.resize(unique(ALL(a)) - a.begin());
}
const int N = 3e5 + 69;
int n, u, v;
vector<int> graph[N];
int go(int u, int p){
vector<int> child;
for(int v: graph[u]) if (v != p){
int cur = go(v, u);
child.push_back(cur);
}
sort(ALL(child));
int ma = 0;
int cnt = 0;
while(child.size()){
maximize(ma, child.back() + ++cnt);
child.pop_back();
}
return ma;
}
int parent[N];
void dfs(int u, int p){
parent[u] = p;
for(int v: graph[u]) if (v != p)
dfs(v, u);
}
vector<int> get_chain(int u, int v){
vector<int> ans;
dfs(u, 0);
while(v != u){
ans.push_back(v);
v = parent[v];
}
ans.push_back(u);
reverse(ALL(ans));
return ans;
}
int solve(){
cin >> n >> u >> v;
for(int i = 1; i< n; ++i){
int u, v; cin >> u >> v;
graph[u].push_back(v);
graph[v].push_back(u);
}
if (u == v)
return go(u, 0);
vector<int> chain = get_chain(u, v);
vector<bool> on_chain(n+1);
for(int i: chain) on_chain[i] = true;
int m = chain.size();
vector<vector<int>> jack(m);
for(int i = 0; i < m; ++i){
int u = chain[i];
for(int v: graph[u]) if (!on_chain[v]){
int cur = go(v, u);
jack[i].push_back(cur);
}
sort(ALL(jack[i]), greater<int>());
}
int l = 0, r = n;
while(l < r){
int mid = (l + r) >> 1;
int t = 0;
int L = -1, R = m;
for(int i = 0; i < m; ++i){
int e = 0, ma_e = 0;
bool continue_further = true;
for(int j: jack[i]){
e++;
if (j + e + t > mid) {
continue_further = false;
break;
}
else if (j + e + t == mid){
ma_e = e;
}
}
if (continue_further == false) break;
t += ma_e + 1;
L = i;
}
t = 0;
for(int i = m - 1; i >= 0; --i){
int e = 0, ma_e = 0;
bool continue_further = true;
for(int j: jack[i]){
e++;
if (j + e + t > mid) {
continue_further = false;
break;
}
else if (j + e + t == mid){
ma_e = e;
}
}
if (continue_further == false) break;
t += ma_e + 1;
R = i;
}
if (L + 1 >= R) r = mid;
else l = mid + 1;
}
return l;
}
int main(void){
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
clock_t start = clock();
cout << solve() << "\n";
cerr << "Time elapsed: " << clock() - start << "ms!\n";
return 0;
}
