#include <bits/stdc++.h>
#define task "T"
#define all(v) (v).begin(), (v).end()
#define rep(i, l, r) for (int i = (l); i <= (r); ++i)
#define Rep(i, r, l) for (int i = (r); i >= (l); --i)
#define DB(X) { cerr << #X << " = " << (X) << '\n'; }
#define DB1(A, _) { cerr << #A << "[" << _ << "] = " << (A[_]) << '\n'; }
#define DB2(A, _, __) { cerr << #A << "[" << _ << "][" << __ << "] = " << (A[_][__]) << '\n'; }
#define DB3(A, _, __, ___) { cerr << #A << "[" << _ << "][" << __ << "][" << ___ << "] = " << (A[_][__][___]) << '\n'; }
#define PR(A, l, r) { cerr << '\n'; rep(_, l, r) DB1(A, _); cerr << '\n';}
#define SZ(x) ((int)(x).size())
#define pb push_back
#define eb emplace_back
#define pf push_front
#define F first
#define S second
#define by(x) [](const auto& a, const auto& b) { return a.x < b.x; } // sort(arr, arr + N, by(a));
#define next ___next
#define prev ___prev
#define y1 ___y1
#define left ___left
#define right ___right
#define y0 ___y0
#define div ___div
#define j0 ___j0
#define jn ___jn
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using namespace std;
typedef pair<int, int> ii;
typedef pair<ii, int> iii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<ll> vl;
const int N = 3e5 + 2;
int n, A, B, par[N], dp[N];
vi g[N];
void dfs(int u)
{
for (int v : g[u]) if (v != par[u])
{
par[v] = u;
dfs(v);
}
}
void dfs1(int u, int p, int b)
{
dp[u] = 0;
vi mn;
for (int v : g[u]) if (v != p && v != b)
{
dfs1(v, u, b);
mn.eb(dp[v]);
}
if (!mn.empty()) sort(all(mn), greater<int> ());
rep(i, 0, SZ(mn) - 1) dp[u] = max(dp[u], mn[i] + i + 1);
}
int main()
{
#ifdef HynDuf
freopen(task".in", "r", stdin);
//freopen(task".out", "w", stdout);
#else
ios_base::sync_with_stdio(false); cin.tie(nullptr);
#endif
cin >> n >> A >> B;
rep(i, 2, n)
{
int u, v;
cin >> u >> v;
g[u].eb(v);
g[v].eb(u);
}
dfs(A);
int u = B;
vi path;
while (u)
{
path.eb(u);
u = par[u];
}
int l = 0, r = SZ(path) - 2, ans = 1e9;
while (l <= r)
{
int m = (l + r) >> 1;
dfs1(A, 0, path[m]);
dfs1(B, 0, path[m + 1]);
ans = min(ans, max(dp[A], dp[B]));
if (dp[B] < dp[A]) l = m + 1;
else r = m - 1;
}
cout << ans;
return 0;
}
