#include <bits/stdc++.h>
using namespace std;
#define ii pair<int, int>
int dist[300300];
map<ii, ii> mp;
vector<int> adj[300300];
bool vis[300300];
int N, A, B;
ii dfs(int a){
vis[a] = true;
int ret = 0;
set<ii> s;
for(auto x : adj[a]){
if(vis[x] || x == B) continue;
s.insert(dfs(x));
}
int ia = s.size();
for(auto x : s){
mp[make_pair(a, x.second)].first = ia;
mp[make_pair(x.second, a)].first = ia;
ret = max(x.first + ia, ret);
ia--;
}
return make_pair(ret, a);
}
ii dfs2(int a){
vis[a] = true;
int ret = 0;
set<ii> s;
for(auto x : adj[a]){
if(vis[x] || x == A) continue;
s.insert(dfs2(x));
}
int ia = s.size();
for(auto x : s){
mp[make_pair(a, x.second)].second = ia;
mp[make_pair(x.second, a)].second = ia;
ret = max(x.first + ia, ret);
ia--;
}
return make_pair(ret, a);
}
void updist(int a, int w){
vis[a] = true;
dist[a] = min(dist[a], w);
for(auto x : adj[a]){
if(vis[x]) continue;
int tmp = mp[make_pair(a, x)].first;
if(tmp == 0) continue;
updist(x, w + tmp);
}
}
void updist2(int a, int w){
vis[a] = true;
dist[a] = min(dist[a], w);
for(auto x : adj[a]){
if(vis[x]) continue;
int tmp = mp[make_pair(a, x)].second;
if(tmp == 0) continue;
updist2(x, w + tmp);
}
}
int main(){
scanf("%d %d %d", &N, &A, &B);
for(int i = 1, u, v; i < N; i++){
scanf("%d %d", &u, &v);
adj[u].push_back(v);
adj[v].push_back(u);
}
dfs(A);
memset(vis, 0, sizeof vis);
dfs2(B);
memset(vis, 0, sizeof vis);
for(int i = 1; i <= N; i++)
dist[i] = INT_MAX;
updist(A, 0);
memset(vis, 0, sizeof vis);
updist2(B, 0);
int ans = INT_MIN;
for(int i = 1; i <= N; i++){
ans = max(ans, dist[i]);
}
printf("%d", ans);
}
