#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define fi first
#define se second
#define mp make_pair
#define fastIO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
const int N = (int)1e6 + 100;
vector<int> T[N];
int trap;
int mouse;
int dfs(int u, int par){
vector<int> lis;
for(auto x : T[u]){
if(x==par) continue;
lis.push_back(dfs(x,u));
}
sort(lis.begin(),lis.end());
int mx = 0;
if(lis.size() >= 2){
mx = lis[lis.size() - 2];
}
return (int)lis.size() + mx;
}
int main(){
fastIO;
int n;
cin >> n;
cin >> trap >> mouse;
int u, v;
for(int i = 1; i < n; i ++ ){
cin >> u >> v;
T[u].push_back(v);
T[v].push_back(u);
}
cout << dfs(mouse,trap);
return 0;
}
