#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#define FAST_IO ios::sync_with_stdio(false);cin.tie(0);
#define pb push_back
#define all(x) begin(x), end(x)
#define space " "
#define TEST_CASES int a; cin >> a; for (int i = 0; i < a; i++) {solve(); cout << endl;}
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ld;
template<typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
mt19937 rng((uint32_t)chrono::steady_clock::now().time_since_epoch().count());
void solve() {
int n, a, b; cin >> n >> a >> b; a--; b--;
vector<vector<int>> adj(n);
for (int i = 0; i < n - 1; i++) {
int u, v; cin >> u >> v; u--; v--;
adj[u].pb(v); adj[v].pb(u);
}
vector<int> prev(n);
auto find = [&] (auto &&self, int node, int parent) -> void {
for (auto x : adj[node]) {
if (x == parent) {
continue;
}
prev[x] = node;
self(self, x, node);
}
};
find(find, a, -1);
int ind = b;
vector<int> path;
while (ind != a) {
path.pb(ind);
ind = prev[ind];
}
path.pb(a);
reverse(all(path));
vector<int> dp(n);
auto dfs = [&] (auto &&self, int node, int parent, int cut, int d) -> void {
vector<int> dist;
dp[node] = d;
for (auto x : adj[node]) {
if (x == parent || node == path[cut] && x == path[cut + 1] || node == path[cut + 1] && x == path[cut]) {
continue;
}
self(self, x, node, cut, d + 1);
dist.pb(dp[x]);
}
sort(all(dist), greater<>());
for (int i = 0; i < dist.size(); i++) {
dp[node] = max(dp[node], dist[i] + i);
}
};
int lo = 0; int hi = path.size() - 2;
while (lo < hi) {
dp.clear(); dp.resize(n);
int mid = (lo + hi + 1) / 2;
dfs(dfs, a, -1, mid, 0);
dfs(dfs, b, -1, mid, 0);
if (dp[a] <= dp[b]) {
lo = mid;
}
else {
hi = mid - 1;
}
}
dp.clear(); dp.resize(n);
dfs(dfs, a, -1, lo, 0);
dfs(dfs, b, -1, lo, 0);
cout << max(dp[a], dp[b]);
}
int main() {
FAST_IO;
//freopen("berries.in", "r", stdin);
//freopen("berries.out", "w", stdout);
//TEST_CASES;
solve(); cout << endl;
/*int a; cin >> a;
for (int i = 1; i <= a; i++){
cout << "Case #" << i << ": ";
solve();
cout << endl;
}*/
}
