#include <bits/stdc++.h>
#define pb push_back
#define all(a) begin(a), end(a)
using namespace std;
int const N = 1000005;
int n, t, m;
vector<vector<int>> adj;
int st = 0;
void load() {
cin >> n >> t >> m;
--t; --m;
adj.resize(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);
}
for (auto u : adj[t]) {
if (u == m) {
st = 2;
}
}
}
int dfs(int ver, int par) {
int ans = adj[ver].size() - 1;
vector<int> tmp;
for (auto u : adj[ver]) if (u != par) {
tmp.pb(dfs(u, ver));
}
sort(all(tmp), greater<>());
if (size(tmp) >= 2) {
ans += tmp[1];
}
return ans;
}
void solve() {
assert(st == 2);
cout << dfs(m, t) << '\n';
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
load();
solve();
return 0;
}
