#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pii;
#define all(x) x.begin(), x.end()
#define sz(x) ((int) ((x).size()))
#define pb push_back
#define F first
#define S second
#define FIO ios_base::sync_with_stdio(false); cin.tie(0)
const int N = 3e5 + 10;
int n, x, y;
vector<int> adj[N];
int par[N];
map<pii, ll> c;
void dfs(int node) {
for (int u : adj[node]) {
if (u == par[node]) continue;
par[u] = node;
dfs(u);
}
}
ll comp(ll node, int s, int prev = 0) {
if (prev == 0 and c.find({node, s}) != c.end()) return c[{node, s}];
vector<ll> se;
for (int u : adj[node]) {
if (u == prev or u == s) continue;
se.pb(comp(u, s, node));
}
sort(all(se));
reverse(all(se));
ll ans = 0;
for (int i = 0; i < sz(se); i++) {
ans = max(ans, i + 1 + se[i]);
}
if (prev == 0) c[{node, s}] = ans;
return ans;
}
int main() {
FIO;
cin >> n >> x >> y;
for (int i = 0; i < n - 1; i++) {
int a, b;
cin >> a >> b;
adj[a].pb(b);
adj[b].pb(a);
}
dfs(x);
vector<int> p;
int cur = y;
while (cur != x) {
p.pb(cur);
cur = par[cur];
}
p.pb(x);
int l = 0;
int r = sz(p) - 2;
ll ans = 1e18;
if (n > 1e3) {
while (l + 2 < r) {
int mid1 = l + (r - l + 1) / 3;
int mid2 = l + (r - l + 1) / 3 * 2;
ll v1 = max(comp(y, p[mid1 + 1]), comp(x, p[mid1]));
ll v2 = max(comp(y, p[mid2 + 1]), comp(x, p[mid2]));
ll vl = max(comp(y, p[l + 1]), comp(x, p[l]));
ll vr = max(comp(y, p[r + 1]), comp(x, p[r]));
if (vl >= v1 and v2 <= vr) {
l = mid1;
r = mid2;
} else if (v1 > v2) {
l = mid1;
} else {
r = mid2;
}
}
}
for (int i = l; i <= r; i++) {
ans = min(ans, max(comp(y, p[i + 1]), comp(x, p[i])));
}
cout << ans << "\n";
return 0;
}
