#include <bits/stdc++.h>
#define fi first
#define se second
#define mp make_pair
#define TASK ""
#define bit(x) (1LL << (x))
#define getbit(x, i) (((x) >> (i)) & 1)
#define ALL(x) (x).begin(), (x).end()
using namespace std;
template <typename T1, typename T2> bool mini(T1 &a, T2 b) {
if (a > b) {a = b; return true;} return false;
}
template <typename T1, typename T2> bool maxi(T1 &a, T2 b) {
if (a < b) {a = b; return true;} return false;
}
mt19937_64 rd(chrono::steady_clock::now().time_since_epoch().count());
int rand(int l, int r) {
return l + rd() % (r - l + 1);
}
const int N = 1e6 + 5;
const int oo = 1e9;
const long long ooo = 1e18;
const int mod = 1e9 + 7; // 998244353;
const long double pi = acos(-1);
vector <int> adj[N];
vector <int> a;
int deg[N];
int par[N];
int dp[N];
int n,t,s;
bool flag[N];
void dfs(int u, int p = -1) {
par[u] = p;
for (int v : adj[u])
if (v != p)
dfs(v, u);
}
void solve(int u, int p = -1) {
deg[u] = 0;
pair <int, int> pir = mp(0, 0);
for (int v : adj[u]) if (v != p) {
solve(v, u);
deg[u]++;
if (maxi(pir.se, dp[v])) {
if (pir.fi < pir.se)
swap(pir.fi, pir.se);
}
}
dp[u] = pir.se + deg[u];
}
bool check(int lim) {
int tot = 0;
for (int v : a)
tot += deg[v];
if (lim < tot)
return false;
for (int i = 0, cnt = 0; i < (int) a.size(); i++) {
int u = a[i];
int skipped = 0;
for (int v : adj[u]) if (!flag[v]) {
skipped += (dp[v] + tot) <= lim;
cnt += (dp[v] + tot > lim);
}
tot -= skipped;
if (cnt > i + 1)
return false;
}
return true;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
#ifdef ONLINE_JUDGE
freopen("mousetrap.inp","r",stdin);
freopen("mousetrap.out","w",stdout);
#endif
cin >> n >> t >> s;
for (int i = 1; i < n; i++) {
int u,v;
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
dfs(s);
while (t != -1) {
a.push_back(t);
t = par[t];
}
reverse(ALL(a));
for (int v : a)
flag[v] = true;
a.pop_back();
for (int u : a) {
deg[u] = 0;
for (int v : adj[u]) if (!flag[v]) {
solve(v, u);
deg[u]++;
// cerr << u << " " << v << "\n";
}
}
int lo = -1;
int hi = n;
while (hi - lo > 1) {
int mid = (lo + hi) >> 1;
if (check(mid))
hi = mid;
else
lo = mid;
}
cout << hi;
return 0;
}
