// IamHereForFun //
#include <bits/stdc++.h>
using namespace std;
const int N = 3e5 + 5;
const int M = 1e2 + 5;
const int LG = 20;
const int S = 1e5 + 5;
const int P = 37;
const int INF = 1e9 + 5;
const int MOD = 1e9 + 7;
int n, st1, st2, dp[N], res;
vector<int> adj[N], path;
bool dfs1(int c, int p)
{
if (c == st2)
{
path.push_back(c);
return true;
}
for (int i : adj[c])
{
if (i == p)
continue;
if (dfs1(i, c))
{
path.push_back(c);
return true;
}
}
return false;
}
void dfs2(int c, int p, int id)
{
dp[c] = 0;
vector<int> v;
for (int i : adj[c])
{
if (i == p)
continue;
if (i == path[id])
continue;
dfs2(i, c, id);
v.push_back(dp[i]);
}
sort(v.rbegin(), v.rend());
for (int x = 0; x < v.size(); x++)
{
v[x] += x + 1;
dp[c] = max(dp[c], v[x]);
}
}
void solve()
{
cin >> n >> st1 >> st2;
if (st1 > st2)
swap(st1, st2);
for (int x = 0; x < n - 1; x++)
{
int a, b;
cin >> a >> b;
adj[a].push_back(b);
adj[b].push_back(a);
}
dfs1(st1, 0);
reverse(path.begin(), path.end());
// for(int i : path) cout << i << " ";
// cout << "\n";
int l = 0, r = path.size() - 2, ans = 0;
while (l <= r)
{
int m = (l + r) / 2;
dfs2(st1, 0, m + 1);
dfs2(st2, 0, m);
// cout << "\n";
if (dp[st1] <= dp[st2])
{
l = m + 1;
ans = m;
}
else
{
r = m - 1;
}
}
// cout << ans << "\n";
dfs2(st1, 0, ans + 1);
dfs2(st2, 0, ans);
// cout << "\n";
// for (int x = 1; x <= n; x++)
// {
// cout << dp[x] << " " << x << "\n";
// }
res = max(dp[st1], dp[st2]);
if (ans + 2 < path.size())
{
dfs2(st1, 0, ans + 2);
dfs2(st2, 0, ans + 1);
res = min(res, max(dp[st1], dp[st2]));
}
cout << res;
}
signed main()
{
// freopen("CP.INP", "r", stdin);
// freopen("CP.OUT", "w", stdout);
// freopen("promote.in", "r", stdin);
// freopen("promote.out", "w", stdout);
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
// cin >> t;
for (int x = 1; x <= t; x++)
{
solve();
}
return 0;
}
