#include <bits/stdc++.h>
using namespace std;
const int N = 1'000'000 + 10;
int n, t, m;
vector<int> ad[N];
vector<int> path;
int f[N];
bool inPath[N];
void dfs(int u, int p = -1) {
inPath[u] = (u == t);
for (const auto& v : ad[u]) {
if (v == p) continue;
dfs(v, u);
inPath[u] |= inPath[v];
}
if (inPath[u]) path.push_back(u);
vector<int> best{0, 0};
for (const auto& v : ad[u]) {
if (v == p || inPath[v]) continue;
best.push_back(f[v]);
}
sort(best.begin(), best.end(), greater<>());
f[u] = best.size() + best[1] - 2;
}
int32_t main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n >> t >> m;
for (int i = 1; i < n; ++i) {
int u, v; cin >> u >> v;
ad[u].push_back(v);
ad[v].push_back(u);
}
vector<pair<int, int>> value;
{ // build value
dfs(m);
reverse(path.begin(), path.end());
path.pop_back();
int it = 0;
for (const auto& u : path) {
it += 1;
sort(ad[u].begin(), ad[u].end(), [&](const auto& a, const auto& b) {
return f[a] > f[b];
});
for (const auto& v : ad[u]) {
if (!inPath[v]) value.push_back({f[v], it});
}
}
for (int i = 0; i < (int)value.size(); ++i)
value[i].first += value.size() - i;
}
auto chk = [&](int mid) {
int it = 0;
for (const auto& [w, num] : value) {
if (it + w > mid) {
if (it >= num) return false;
it += 1;
}
}
return it <= mid;
};
int l = 0, r = 2 * n, answer = 0;
while (l <= r) {
int mid = (l + r) >> 1;
if (chk(mid)) r = (answer = mid) - 1;
else l = mid + 1;
}
cout << answer << "\n";
}
