#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define ll long long
#define mp make_pair
#define pb push_back
#define pii pair<int, int>
#define pll pair<ll, ll>
#define MASK(i) (1LL << (i))
#define BIT(x, i) (((x) >> (i)) & 1)
#define all(x) (x).begin() , (x).end()
#define TIME (1.0 * clock() / CLOCKS_PER_SEC)
#define file "name"
template <typename T1, typename T2> bool minimize(T1 &a, T2 b){if (a > b) {a = b; return true;} return false;}
template <typename T1, typename T2> bool maximize(T1 &a, T2 b){if (a < b) {a = b; return true;} return false;}
const int inf = 1e9;
const ll linf = 1e17;
const int mod = 1e9 + 7;
const int N = 1e6 + 5;
int n, s, t;
int en[2][N];
vector <int> a[N];
vector <int> p;
bool check;
int res = inf;
int f[N];
void inp()
{
cin >> n >> s >> t;
for(int i = 1; i < n; ++i) {
int u, v;
cin >> u >> v;
a[u].pb(v);
a[v].pb(u);
}
}
void dfs_pre(int u, int pre) {
if(u == t) check = 1;
for(int v : a[u]) {
if(v == pre || check) continue;
dfs_pre(v, u);
}
if(check) p.pb(u);
}
int dfs(int u, int pre, bool type) {
f[u] = 0;
vector <int> x;
for(int v : a[u]) {
if(v == pre || en[type][v]) continue;
x.pb(dfs(v, u, type));
}
sort(x.begin(), x.end(), greater <int>());
for(int i = 0; i < x.size(); ++i) maximize(f[u], x[i] + i + 1);
return f[u];
}
bool cal(int k) {
en[1][p[k]] = 1;
en[0][p[k + 1]] = 1;
int cx = dfs(s, 0, 0);
int cy = dfs(t, 0, 1);
// cout << cx << ' ' << cy << '\n';
minimize(res, max(cx, cy));
en[1][p[k]] = 0;
en[0][p[k + 1]] = 0;
return cx <= cy;
}
void solve()
{
dfs_pre(s, s);
reverse(p.begin(), p.end());
cal(0);
int l = 0, r = p.size() - 2, id = 1;
while(l <= r) {
int m = (l + r) >> 1;
if(cal(m)) {
l = m + 1;
id = m;
}
else r = m - 1;
}
if(id + 1 < p.size()) cal(id + 1);
cout << res;
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
// freopen(file".inp" , "r" , stdin);
// freopen(file".out" , "w" , stdout);
inp();
solve();
return 0;
}
