#include <bits/stdc++.h>
using namespace std;
#define int int64_t
const int INF = 1e18;
int n, a, b;
vector<int> del;
vector<vector<array<int, 2>>> g;
vector<int> path_edge_indices;
bool find_path_to_b(int u, int p) {
if (u == b) return true;
for (auto [v, ind] : g[u]) {
if (v != p && !del[ind] && find_path_to_b(v, u)) {
path_edge_indices.push_back(ind);
return true;
}
}
return false;
}
int calc_cost(int u, int p) {
vector<int> costs;
for (auto [v, ind] : g[u])
if (v != p && !del[ind]) costs.push_back(calc_cost(v, u));
sort(costs.rbegin(), costs.rend());
int cost = 0;
for (int i = 0; i < costs.size(); ++i)
cost = max(cost, i + 1 + costs[i]);
return cost;
}
void solve() {
cin >> n >> a >> b;
a--, b--;
g.resize(n), del.resize(n - 1);
for (int i = 0, u, v; i < n - 1; ++i) {
cin >> u >> v, --u, --v;
g[u].push_back({v, i});
g[v].push_back({u, i});
}
if (a == b) {
cout << calc_cost(a, -1) << '\n';
return;
}
int ans = INF;
find_path_to_b(a, -1);
for (int ind : path_edge_indices) {
del[ind] = true;
ans = min(ans, max(calc_cost(a, -1), calc_cost(b, -1)));
del[ind] = false;
}
cout << ans << '\n';
}
signed main() {
ios::sync_with_stdio(0), cin.tie(0), solve();
}
