#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
const int MAXN = 3e5 + 7;
const int LOG = 18;
vector<pair<int, int>> G[MAXN];
vector<int> edges;
int wsk[MAXN];
int depth[MAXN];
int dp[MAXN];
bool blocked[MAXN];
int parent[MAXN];
int root1, root2;
int res;
int n;
bool cmp(pair<int ,int> a, pair<int, int> b){
return dp[a.first] > dp[b.first];
}
void dfs(int v, int p, int d = 0){
depth[v] = d;
parent[v] = p;
for(auto [u, id] : G[v]){
if(u == p) continue;
wsk[u] = id;
dfs(u, v, d+1);
}
}
void dfs_res(int v, int p){
dp[v] = 0;
for(auto [u, id] : G[v]){
if(u == root1 || u == root2 || blocked[id] || u == p) continue;
dfs_res(u, v);
}
sort(G[v].begin(), G[v].end(), cmp);
int add = 1;
for(auto [u, id] : G[v]){
if(u == root1 || u == root2 || blocked[id] || u == p) continue;
dp[v] = max(dp[v], dp[u] + add);
add++;
}
}
void BS(){
int l = 0, r = edges.size(), mid;
while(l < r){
mid = (l + r)/2;
blocked[wsk[edges[mid]]] = true;
dfs_res(root1, root1);
dfs_res(root2, root2);
if(dp[root2] <= dp[root1]) l = mid + 1;
else r = mid;
res = min(res, max(dp[root1], dp[root2]));
blocked[wsk[edges[mid]]] = false;
}
}
void solve(){
int v = root1;
int u = root2;
while(depth[v] < depth[u]){
edges.push_back(u);
u = parent[u];
}
BS();
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n;
cin >> root1 >> root2;
for(int i = 1; i < n; i++){
int a, b;
cin >> a >> b;
G[a].push_back({b, i});
G[b].push_back({a, i});
}
dfs(root1, root1);
dfs_res(root1, root1);
dfs_res(root2, root2);
res = max(dp[root1], dp[root2]);
/*
for(int i = 1; i <= n; i++){
blocked[wsk[i]] = true;
dfs_res(root1, root1);
dfs_res(root2, root2);
res = min(res, max(dp[root1], dp[root2]));
blocked[wsk[i]] = false;
}
*/
solve();
cout << res << endl;
}
