#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];
bool 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;
}
}
}
int dp[100009];
void calc(int v, int p){
vector<int> vec;
for(auto u : g[v]){
if(u == p)
continue;
calc(u, v);
vec.pb(dp[u]);
}
sort(vec.begin(), vec.end(), greater<int>());
for(int i = 0; i < vec.size(); ++i)
dp[v] = max(dp[v], i+1+vec[i]);
}
int dis[100009];
priority_queue<pii> pq;
int check(int time){
for(int i = 1; i <= n; ++i)
dis[i] = 1e9;
while(!pq.empty())pq.pop();
pq.push({0, a});
pq.push({0, b});
dis[a] = dis[b] = 0;
while(!pq.empty()){
int v = pq.top().ss;
int curdis = -pq.top().ff;
pq.pop();
if(curdis > dis[v])
continue;
if(curdis > time)
return 0;
vector<int> vec;
for(auto u : g[v]){
if(oth[u])
continue;
vec.pb(dp[u]);
}
sort(vec.begin(), vec.end(), greater<int>());
int cur = 2;
int ret = 1;
for(auto u : vec){
if(curdis + u + cur == time+1){
ret = cur;
}
else if(curdis + u + cur > time)
return 0;
++cur;
}
for(auto u : g[v])
if(oth[u] && dis[u] > curdis+ret){
dis[u] = curdis+ret;
pq.push({-dis[u], u});
}
}
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);
}
oth[b] = 1;
dfs1(a, -1);
for(int i = 1; i <= n; ++i){
if(oth[i] == 0) continue;
for(auto u : g[i]){
if(oth[u] == 0)
calc(u, i);
}
}
int l = 0, r = 300009;
while(l < r){
int m = l+(r-l)/2;
if(check(m))
r = m;
else
l = m+1;
}
cout << l << "\n";
}
