#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
template<typename T> using ordered_set = tree<T,null_type,less_equal<T>,rb_tree_tag,
tree_order_statistics_node_update>;
#define ll long long
#define pi pair<int,int>
#define vi vector<int>
#define pb push_back
#define all(a) a.begin(),a.end()
const int mxn = 3e5+1;
vector<int> g[mxn];
int par[mxn],ver[mxn];
int dp[mxn], memo[mxn];
int v1,v2;
void pre_dfs(int u, int p){
par[u] = p;
for(auto v : g[u]) if(v!=p)
pre_dfs(v,u);
}
void dfs(int u, int p){
dp[u] = 0;
vector<int> vec;
for(auto v : g[u]) if(v!=p && v!=v1){
dfs(v,u); vec.push_back(dp[v]);
}
int cnt = vec.size();
sort(all(vec));
for(int i = 0; i < cnt; i++)
dp[u] = max(dp[u],vec[i]+cnt-i);
}
void solve(){
int n,a,b; cin >> n >> a >> b;
if(n==2){
cout << 0; return;
}
for(int i = 1; i < n; i++){
int u,v; cin >> u >> v;
g[u].pb(v); g[v].pb(u);
}
pre_dfs(a,a);
int l = 0, r = 0; ver[0] = b;
int u = b;
while(par[u]!=a){
u = par[u]; ver[++r] = u;
}
ver[r+1] = a;
if(n <= 1000){
int ans = n+1;
for(int i = 0; i <= r; i++){
v1 = ver[i]; dfs(a,a);
v1 = ver[i+1]; dfs(b,b);
ans = min(ans,max(dp[a],dp[b]));
}
cout << ans; return;
}
while(l+3 < r){
int m1 = l + (r-l)/3;
int m2 = r - (r-l)/3;
int r1 = memo[m1];
if(r1==0){
v1 = ver[m1]; dfs(a,a);
v1 = ver[m1+1];dfs(b,b);
r1 = memo[m1] = max(dp[a],dp[b]);
}
int r2 = memo[m2];
if(r2==0){
v1 = ver[m2]; dfs(a,a);
v1 = ver[m2+1];dfs(b,b);
r2 = memo[m2] = max(dp[a],dp[b]);
}
if(r1 <= r2) r = m2;
else l = m1;
}
int ans = n;
for(int i = l; i <= r; i++){
int r1 = memo[i];
if(r1==0){
v1 = ver[i]; dfs(a,a);
v1 = ver[i+1];dfs(b,b);
r1 = memo[i] = max(dp[a],dp[b]);
}
ans = min(ans,r1);
}
cout << ans;
}
signed main(){
cin.tie(0)->sync_with_stdio(0);
int t = 1;
// cin >> t;
while(t--){solve();}
}
