#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define sp ' '
#define en '\n'
#define smin(a, b) a = min(a, b)
#define smax(a, b) a = max(a, b)
using namespace std;
const int N = 3e5 + 2;
const int inf = 1e9;
vector<pair<int, int>> g[N];
int par[N];
void Dfs(int s, int e) {
for (auto u : g[s]) {
if (u.first == e) continue;
par[u.first] = u.second;
Dfs(u.first, s);
}
}
int Dfs1(int s, int e, int c) {
vector<int> all;
for (auto u : g[s]) {
if (u.first == e || u.second == c) continue;
all.push_back(Dfs1(u.first, s, c));
}
sort(all.rbegin(), all.rend());
int o = 0;
for (int i = 0; i < all.size(); i++) {
smax(o, all[i] + i + 1);
}
return o;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n, a, b;
cin >> n >> a >> b;
par[1000000]++;
vector<int> u(n - 1), v(n - 1);
for (int i = 0; i < n - 1; i++) {
cin >> u[i] >> v[i];
g[u[i]].push_back({v[i], i});
g[v[i]].push_back({u[i], i});
}
Dfs(1, 0);
for (int i = 0; i < n - 1; i++) {
if (par[v[i]] != i) swap(u[i], v[i]);
}
vector<int> p;
int x = b;
while (x != a) {
p.push_back(par[x]);
x = u[par[x]];
}
int sz = p.size();
int l = 0, r = sz - 2, gde = sz - 1;
while (l <= r) {
int mid = l + r >> 1;
int c = p[mid]; int d = p[mid + 1];
if (max(Dfs1(a, 0, c), Dfs1(b, 0, c)) < max(Dfs1(a, 0, d), Dfs1(b, 0, d))) {
l = mid + 1;
gde = mid;
}
else r = mid - 1;
}
cout << max(Dfs1(a, 0, p[gde]), Dfs1(b, 0, p[gde])) << en;
return 0;
}
