#include <bits/stdc++.h>
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<pii, int> piii;
typedef pair<ll, ll> pll;
#define debug(x) cerr<<#x<<'='<<(x)<<endl;
#define debugp(x) cerr<<#x<<"= {"<<(x.first)<<", "<<(x.second)<<"}"<<endl;
#define debug2(x, y) cerr<<"{"<<#x<<", "<<#y<<"} = {"<<(x)<<", "<<(y)<<"}"<<endl;
#define debugv(v) cerr<<#v<<" : ";for (auto x:v) cerr<<x<<' ';cerr<<endl;
#define all(x) x.begin(), x.end()
#define pb push_back
#define kill(x) return cout<<x<<'\n', 0;
const ld eps=1e-7;
const int inf=1000000010;
const ll INF=10000000000000010LL;
const int mod = 1000000007;
const int MAXN = 500010;
int n, m, k, q, u, v, x, y, t, a, b, len;
int dp[MAXN];
bool on_path[MAXN];
vector<int> G[MAXN];
vector<int> path;
bool dfs_path(int node, int par){
path.pb(node);
if (node==b) return 1;
for (int v:G[node]) if (v!=par && dfs_path(v, node)) return 1;
path.pop_back();
return 0;
}
int dfs_dp(int node, int par){
vector<int> vec;
for (int v:G[node]) if (!on_path[v] && v!=par) vec.pb(dfs_dp(v, node));
sort(all(vec));
reverse(all(vec));
for (int i=0; i<vec.size(); i++) dp[node]=max(dp[node], vec[i]+i+1);
return dp[node];
}
int check(int val){
int cnt=0, tmp=val;
for (int i=0; i<=len; i++){
if (dp[path[i]]<tmp) tmp--, cnt++;
else if (dp[path[i]]==tmp) {cnt++; break ;}
else break ;
}
tmp=val;
for (int i=len; i>=0; i--){
if (dp[path[i]]<tmp) tmp--, cnt++;
else if (dp[path[i]]==tmp) {cnt++; break ;}
else break ;
}
return cnt>=len+1;
}
int main(){
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
//freopen("input.txt", "r", stdin);
//freopen("output.txt", "w", stdout);
cin>>n>>a>>b;
for (int i=1; i<n; i++){
cin>>u>>v;
G[u].pb(v);
G[v].pb(u);
}
dfs_path(a, a);
len=path.size()-1;
for (int v:path) on_path[v]=1;
for (int v:path) dfs_dp(v, v);
int dwn=-1, up=n;
while (up-dwn>1){
int mid=(dwn+up)>>1;
if (check(mid)) up=mid;
else dwn=mid;
}
cout<<up<<'\n';
return 0;
}
/*
6 2 1
1 2
2 3
2 4
1 5
5 6
10 1 2
1 2
2 5
1 3
1 4
4 6
6 7
3 8
3 9
3 10
17 1 2
1 3
1 4
4 6
6 7
3 8
3 9
3 10
1 13
13 5
13 11
13 12
13 14
14 15
15 16
15 17
14 2
*/
