#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 border1, int border2) {
dp[node] = 0;
//cerr << " " << node << "/\n";
for(int i = 0;i < g[node].size();i++) {
int to = g[node][i];
if(to != parent && !((node == border1 && to == border2) || (node == border2 && to == border1))) {
computeDp(to, node, border1, border2);
}
}
vector <int> aux;
for(int i = 0;i < g[node].size();i++) {
int to = g[node][i];
if(to != parent && !((node == border1 && to == border2) || (node == border2 && to == border1))) {
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 sourceA, int sourceB, int border1, int border2) {
int solA, solB;
computeDp(sourceA, sourceA, border1, border2);
solA = dp[sourceA];
computeDp(sourceB, sourceB, border1, border2);
solB = dp[sourceB];
return max(solA, solB);
}
int auxSolve(int sourceA, int sourceB, int border1, int border2) {
int solA, solB;
computeDp(sourceA, sourceA, border1, border2);
solA = dp[sourceA];
computeDp(sourceB, sourceB, border1, border2);
solB = dp[sourceB];
return (solA - solB);
}
int cautbinBorder(int from, int to, int sourceA, int sourceB) {
if(from >= to) {
return from;
}else {
int mid = (from + to) / 2;
int border1 = path[mid-1], border2 = path[mid];
if(auxSolve(sourceA, sourceB, border1, border2) > 0) {
return cautbinBorder(from, mid, sourceA, sourceB);
}else {
return cautbinBorder(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;
int border = cautbinBorder(1, dist, sourceA, sourceB);
int ans = solve(sourceA, sourceB, path[border-1], path[border]);
//cerr << " " << border-1 << ' ' << border << " - " << path[border-1] << ' ' << path[border] << '\n';
if(border > 1) {
ans = min(ans, solve(sourceA, sourceB, path[border-2], path[border-1]));
}
if(border < dist) {
ans = min(ans, solve(sourceA, sourceB, path[border], path[border+1]));
}
cout << ans << '\n';
return 0;
}
