#include <bits/stdc++.h>
using namespace std;
#define forsn(i, s, n) for (int i = int(s); i < int(n); i++)
#define forn(i, n) forsn(i, 0, n)
#define dforsn(i, s, n) for (int i = int(n) - 1; i >= int(s); i--)
#define dforn(i, n) dforsn(i, 0, n)
#define foreach(it, c) for (auto it = begin(c); it != end(c); it++)
#define sz(x) int(x.size())
#define all(x) begin(x), end(x)
#define pb push_back
#define eb emplace_back
using vi = vector<int>;
using ll = long long;
using ld = long double;
using vll = vector<ll>;
using ii = pair<int, int>;
#define fst first
#define snd second
const int MAX_N = 1e6 + 20;
vi adj[MAX_N];
int dp[MAX_N], par[MAX_N];
void dfs0(int u, int p = -1) {
par[u] = p;
for (int v : adj[u]) {
if (v != p) dfs0(v, u);
}
}
void dfs(int u, int p) {
int maxi = 0, sndMaxi = 0;
for (int v : adj[u]) {
if (v == p) continue;
dfs(v, u);
if (dp[v] >= maxi) {
sndMaxi = maxi;
maxi = dp[v];
} else if (dp[v] > sndMaxi) {
sndMaxi = dp[v];
}
}
dp[u] = sndMaxi + sz(adj[u]) - 1;
}
int main() {
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n, t, m;
cin >> n >> t >> m;
--t, --m;
forn(i, n - 1) {
int u, v;
cin >> u >> v;
--u, --v;
adj[u].pb(v);
adj[v].pb(u);
}
dfs0(t);
vi path{m};
while (path.back() != t) path.pb(par[path.back()]);
vector<bool> inPath(n);
for (int u : path) inPath[u] = true;
vector<vi> l;
l.reserve(sz(path));
for (int u : path) {
vi vals;
for (int v : adj[u]) {
if (inPath[v]) continue;
dfs(v, u);
vals.pb(dp[v]);
}
sort(all(vals), greater<int>());
l.pb(vals);
}
vi suff(sz(path));
suff[sz(path) - 1] = 0;
dforn(i, sz(path) - 1) suff[i] = suff[i + 1] + sz(l[i]);
auto check = [&](int moves) {
int toUse = 1;
forn(i, sz(path) - 1) {
int blocked = 0;
for (auto val : l[i]) {
if (val + suff[i] - blocked > moves) {
if (!toUse || !moves) return false;
toUse--, moves--, blocked++;
}
}
toUse++;
}
return true;
};
int lo = -1, hi = 1e9;
while (hi - lo > 1) {
int mid = (lo + hi) / 2;
if (check(mid)) hi = mid;
else lo = mid;
}
cout << hi << '\n';
return 0;
}
