#include <bits/stdc++.h>
using namespace std;
template <class a, class b> ostream& operator<<(ostream &l, const pair<a, b> &r) {
l << "(" << r.first << ", " << r.second << ")" << endl;
}
template <class t> ostream& operator<<(ostream &l, const vector<t> &r) {
l << "{";
for (int i = 0; i + 1 < r.size(); i++) l << r[i] << ", ";
if (!r.empty()) l << r.back();
l << "}";
return l;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
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].push_back(v);
adj[v].push_back(u);
}
vector<int> par(n);
auto marks = [&](int v, int p, auto &&self) -> void {
par[v] = p;
for (int i : adj[v]) {
if (i != p) {
self(i, v, self);
}
}
};
marks(a, -1, marks);
vector<int> path;
int ptr = b;
do {
path.push_back(ptr);
ptr = par[ptr];
} while (ptr != a);
path.push_back(a);
vector<int> dp(n, 1e6);
int banned = -1;
auto dfs = [&](int v, int p, auto &&self) -> void {
for (int i : adj[v]) {
if (i != p && i != banned) {
self(i, v, self);
}
}
vector<int> unmarks;
unmarks.reserve(adj[v].size() - 1);
for (int i : adj[v]) {
if (i != p && i != banned) {
unmarks.push_back(dp[i]);
}
}
sort(unmarks.begin(), unmarks.end());
reverse(unmarks.begin(), unmarks.end());
int time = 0;
// greedily assign longest child times to earliest copy
for (int i = 0; i < unmarks.size(); i++) {
time = max(time, i + 1 + unmarks[i]);
}
dp[v] = time;
};
auto eval = [&](int x) -> pair<int, int> {
banned = path[x];
dfs(b, -1, dfs);
int bCost = dp[b];
banned = path[x - 1];
dfs(a, -1, dfs);
int aCost = dp[a];
// cout << "eval " << x << " " << aCost << " " << bCost << endl;
return {aCost, bCost};
};
int lo = 0, hi = path.size() - 1;
// last one s.t. a part is strictly greater than b part
while (lo < hi) {
int mid = lo + (hi - lo + 1) / 2;
pair<int, int> costs = eval(mid);
if (costs.first > costs.second) {
lo = mid;
} else {
hi = mid - 1;
}
}
int ans = 1e6;
if (lo > 0) {
pair<int, int> cur = eval(lo);
ans = max(cur.first, cur.second);
}
if (lo < path.size() - 1) {
pair<int, int> cur = eval(lo + 1);
ans = min(ans, max(cur.first, cur.second));
}
// cout << path << endl;
// cout << lo << endl;
cout << ans << '\n';;
}
