#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int const INF = 1e9+7;
int const NMAX = 3e5;
int n;
vector <int> g[1 + NMAX];
int dist;
vector <int> path;
bool buildPath(int node, int parent, int target) {
if(node == target) {
path.push_back(node);
return true;
}
for(int i = 0;i < g[node].size();i++) {
int to = g[node][i];
if(to != parent) {
if(buildPath(to, node, target)) {
path.push_back(node);
return true;
}
}
}
return false;
}
int dp[1 + NMAX];
void computeDp(int node, int parent, int border) {
dp[node] = 0;
//cerr << " " << node << "/\n";
for(int i = 0;i < g[node].size();i++) {
int to = g[node][i];
if(to != parent && to != border) {
computeDp(to, node, border);
}
}
vector <int> aux;
for(int i = 0;i < g[node].size();i++) {
int to = g[node][i];
if(to != parent && to != border) {
aux.push_back(dp[to]);
}
}
if(aux.size() > 0) {
sort(aux.begin(), aux.end());
int add = aux.size();
for(int i = 0;i < aux.size();i++) {
dp[node] = max(dp[node], aux[i]+add);
add--;
}
}
//cerr << " " << node << " : " << dp[node] << "\\\n";
}
int solve(int root, int borderDist) {
int border;
if(root == path[0]) {
border = path[borderDist];
}else {
border = path[path.size()-borderDist-1];
}
//cerr << root << " :\n";
computeDp(root, root, border);
return dp[root];
}
int cautbinLimit(int from, int to, int root, int limit) {
if(from >= to) {
if(solve(root, from) <= limit) {
return from;
}else {
return -INF;
}
}else {
int mid = (from + to + 1) / 2;
if(solve(root, mid) <= limit) {
return cautbinLimit(mid, to, root, limit);
}else {
return cautbinLimit(from, mid-1, root, limit);
}
}
}
int cautbin(int from, int to, int sourceA, int sourceB) {
if(from >= to) {
return from;
}else {
int mid = (from + to) / 2;
int limitA = cautbinLimit(1, dist, sourceA, mid);
int limitB = cautbinLimit(1, dist, sourceB, mid);
if(limitA + limitB >= dist+1) {
return cautbin(from, mid, sourceA, sourceB);
}else {
return cautbin(mid+1, to, sourceA, sourceB);
}
}
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
int sourceA, sourceB;
cin >> n >> sourceA >> sourceB;
for(int i = 1;i < n;i++) {
int a, b;
cin >> a >> b;
g[a].push_back(b);
g[b].push_back(a);
}
buildPath(sourceB, sourceB, sourceA);
dist = path.size()-1;
cout << cautbin(1, n, sourceA, sourceB);
return 0;
}
