#include <bits/stdc++.h>
#define time() cerr << 1.0 * clock() / CLOCKS_PER_SEC << "s\n"
#define fi first
#define se second
#define pb push_back
#define FOR(i, a, b) for (int i = a, _b = b; i <= _b; ++i)
#define FORD(i, a, b) for (int i = a, _b = b; i >= _b; --i)
#define FORLL(i, a, b) for (ll i = a, _b = b; i <= _b; ++i)
#define FORDLL(i, a, b) for (ll i = a, _b = b; i >= _b; --i)
#define all(x) x.begin(), x.end()
#define uni(x) sort(all(x)), x.erase(unique(all(x)), x.end())
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define dbg(...) debug(#__VA_ARGS__, __VA_ARGS__)
template<typename T>
void __print_one(const char *&s, const T &x)
{
while (*s == ' ') ++s;
const char *p = s;
int bal = 0;
while (*s)
{
if (*s == '(') ++bal;
else if (*s == ')') --bal;
else if (*s == ',' && bal == 0) break;
++s;
}
cerr.write(p, s - p) << " = " << x;
if (*s == ',')
{
cerr << " , ";
++s;
}
}
template<typename... Args>
void debug(const char *s, Args... args)
{
cerr << "[ ";
int dummy[] = { 0 , (__print_one(s, args), 0)... };
(void)dummy;
cerr << " ]\n\n";
}
template<class X>
bool maximize(X &a, X b)
{
if (a < b)
{
a = b;
return true;
}
return false;
}
template<class X>
bool minimize(X &a, X b)
{
if (a > b)
{
a = b;
return true;
}
return false;
}
// -----------------------------------------------------------------
const int maxn = 3e5 + 3;
int n, root1, root2, dp[maxn], dptwo[maxn], cam1, cam2;
vector<int> vecs;
vector<int> g[maxn];
// -----------------------------------------------------------------
bool dfs_pre(int u, int p)
{
bool cur = u == root2;
for (int v : g[u])
{
if (v == p) continue;
bool ok = dfs_pre(v, u);
cur |= ok;
if (ok)
vecs.push_back(v);
}
return cur;
}
void dfs(int u, int p, int *dp, int cam)
{
dp[u] = 0;
vector<int> vec;
for (int v : g[u])
{
if (v == p || v == cam) continue;
dfs(v, u, dp, cam);
vec.push_back(dp[v]);
}
sort(all(vec), greater<int>());
int cnt = 0;
for (int x : vec)
dp[u] = max(dp[u], x + (++cnt));
}
ll f(int x)
{
cam1 = -1, cam2 = -1;
if (x < int(vecs.size()) - 1) cam1 = vecs[x + 1];
if (x >= 0) cam2 = vecs[x];
dfs(root1, -1, dp, cam1);
dfs(root2, -1, dptwo, cam2);
return max(dp[root1], dptwo[root2]);
}
void solve()
{
cin >> n >> root1 >> root2;
FOR(i, 2, n)
{
int u, v; cin >> u >> v;
if (u > v) swap(u, v);
g[u].push_back(v);
g[v].push_back(u);
}
dfs_pre(root1, -1);
vecs.push_back(root1);
reverse(all(vecs));
// for (int x : vecs)
// dbg(x);
int l = 0, r = (int)vecs.size() - 2;
while (l < r)
{
int mid = l + r >> 1;
if (f(mid) > f(mid + 1))
l = mid + 1;
else
r = mid;
}
cout << f(l);
}
signed main()
{
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define TASK "test"
if (fopen(TASK".INP", "r"))
{
freopen(TASK".INP", "r", stdin);
freopen(TASK".OUT", "w", stdout);
}
solve();
return 0;
}
