#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define endl '\n'
#define MASK(i) (1LL << (i))
#define ull unsigned long long
#define ld long double
#define pb push_back
#define all(x) (x).begin() , (x).end()
#define BIT(x , i) ((x >> (i)) & 1)
#define TASK "task"
#define sz(s) (int) (s).size()
using namespace std;
const int mxN = 1e6 + 227;
const int inf = 1e9 + 277;
const int mod = 1e9 + 7;
const ll infll = 1e18 + 7;
const int base = 200;
const int LOG = 20;
template <typename T1, typename T2> bool minimize(T1 &a, T2 b) {
if (a > b) {a = b; return true;} return false;
}
template <typename T1, typename T2> bool maximize(T1 &a, T2 b) {
if (a < b) {a = b; return true;} return false;
}
int n;
int t;
int m;
int deg[mxN];
int dp[mxN];
int par[mxN];
bool spec[mxN];
vector<int> adj[mxN];
vector<int> path;
void dfs(int u, int p)
{
for(int v : adj[u]) {
if(v == p) continue;
par[v] = u;
dfs(v, u);
}
}
void calcdp(int u, int p)
{
pair<int, int> best = {0, 0};
for(int v : adj[u]) {
if(v == p) continue;
deg[u]++;
calcdp(v, u);
int tmp = best.fi;
if(maximize(best.fi, dp[v]) == true) best.se = tmp;
else maximize(best.se, dp[v]);
}
dp[u] = best.se + deg[u];
}
bool ok(int x)
{
int sumdeg = 0;
for(auto it : path) sumdeg += deg[it];
if(sumdeg > x) return false;
int cur = 0;
int need = 0;
for(auto u : path) {
++cur;
int re = 0;
for(int v : adj[u]) {
if(spec[v] == true) continue;
re += (dp[v] + sumdeg) <= x;
need += (dp[v] + sumdeg) > x;
}
sumdeg -= re;
if(need > cur) return false;
}
return true;
}
void solve()
{
cin >> n >> t >> m;
for(int i = 1; i < n; i++) {
int u, v;
cin >> u >> v;
adj[u].pb(v);
adj[v].pb(u);
}
par[m] = 0;
dfs(m, -1);
int pos = t;
while(pos != 0) {
if(pos != t) path.pb(pos);
pos = par[pos];
}
reverse(all(path));
spec[t] = true;
for(auto it : path) spec[it] = true;
for(auto u : path) {
for(int v : adj[u]) {
if(spec[v] == false) {
calcdp(v, u);
deg[u]++;
}
}
}
// cout << dp[6] << ' ' << dp[7] << ' ' << dp[3] << endl;
int lo = 0, hi = inf;
while(lo + 1 < hi) {
int mid = (lo + hi) / 2;
if(ok(mid) == true) hi = mid;
else lo = mid;
}
if(ok(lo)) cout << lo;
else cout << hi;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
//freopen(TASK".in" , "r" , stdin);
//freopen(TASK".out" , "w" , stdout);
int tc = 1;
// cin >> tc;
while(tc--) {
solve();
}
return 0;
}
