#include <bits/stdc++.h>
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<ull, ull> pull;
typedef pair<int, int> pii;
typedef pair<ld, ld> pld;
int n, a, b;
vector<int> g[100009];
int oth[100009];
void dfs1(int v, int p){
if(oth[v]) return;
for(auto u : g[v]){
if(u == p)
continue;
dfs1(u, v);
if(oth[u]){
oth[v] = 1;
return;
}
}
}
set<pii> s;
int vis[100009];
int dp[100009];
int calc(int v){
if(dp[v] != -1)
return dp[v];
vector<int> vec;
vis[v] = 1;
for(auto u : g[v]){
if(vis[u] == 1)
continue;
vec.pb(calc(u));
}
sort(vec.begin(), vec.end(), greater<int>());
int ret = 0;
for(int i = 0; i < vec.size(); ++i){
ret = max(ret, vec[i]+i+1);
}
return dp[v] = ret;
}
int dis[100009];
int maincalc(int v, int curtime, int time){
if(dis[v] < curtime)
return 0;
vector<int> vec;
vis[v] = 1;
for(auto u : g[v]){
if(vis[u] == 1)
continue;
vec.pb(calc(u));
}
sort(vec.begin(), vec.end(), greater<int>());
int cur = 2;
int ret = 1;
for(auto u : vec){
if(u + cur + curtime > time){
ret = cur;
--cur;
}
if(u + cur + curtime > time){
return 1;
}
++cur;
}
for(auto u : g[v])
if(ret+curtime < dis[u] && oth[u] == 1 && ret+curtime <= time){
vis[u] = 1;
dis[u] = ret+curtime;
s.insert({ret+curtime, u});
}
return 0;
}
int check(int time){
for(int i = 1; i <= n; ++i){
vis[i] = 0;
dis[i] = 1e9;
}
vis[a] = vis[b] = 1;
s.insert({0, a});
s.insert({0, b});
while(!s.empty()){
auto it = s.begin();
if(maincalc((*it).ss, (*it).ff, time))
return 0;
s.erase(it);
}
for(int i = 1; i <= n; ++i)
if(vis[i] == 0 && oth[i] == 1)
return 0;
return 1;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
//freopen("in.txt", "r", stdin);
//freopen("out.txt", "w", stdout);
cin >> n >> a >> b;
for(int i = 1; i < n; ++i){
int t1, t2;
cin >> t1 >> t2;
g[t1].pb(t2);
g[t2].pb(t1);
}
for(int i = 1; i <= n; ++i)
dp[i] = -1;
oth[a] = oth[b] = 1;
dfs1(a, b);
int l = 0, r = 300009;
while(l < r){
int m = (l+r)/2;
if(check(m))
r = m;
else
l = m+1;
}
cout << l << "\n";
}
