#include <bits/stdc++.h>
using namespace std;
#define sz(x) ((int)size(x))
#define all(x) begin(x), end(x)
#define trace(x) cout << #x << ": " << (x) << endl;
typedef long long ll;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
int rand(int l, int r) { return (int) ((ll) rnd() % (r - l + 1)) + l; }
template<typename T>
bool ckmn(T &x, T y) {
if (x > y) {
x = y;
return true;
}
return false;
}
template<typename T>
bool ckmx(T &x, T y) {
if (x < y) {
x = y;
return true;
}
return false;
}
const int N = 1000001, infI = 1e9 + 7;
const ll infL = 3e18;
int n, t, m;
vector<int> g[N];
int dp[N], parent[N];
vector<int> tops;
void dfs(int v, int p) {
parent[v] = p;
int mx1 = 0, mx2 = 0, cnt = 0;
for (int to: g[v]) {
if (to != p) {
++cnt;
dfs(to, v);
if (mx1 < dp[to]) {
mx2 = mx1;
mx1 = dp[to];
} else if (mx2 < dp[to]) {
mx2 = dp[to];
}
}
}
dp[v] = mx2 + cnt;
tops.push_back(v);
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> t >> m;
--t, --m;
if (t == m) {
exit(1);
}
for (int i = 1; i < n; ++i) {
int a, b;
cin >> a >> b;
--a, --b;
g[a].push_back(b);
g[b].push_back(a);
}
dfs(t, t);
int l = -1, r = infI;
while (l + 1 < r) {
int mid = (l + r) >> 1;
int total = 0;
{
int x = m;
int prv = 0;
while (x != t) {
for (int to : g[x]) {
if (to != prv && to != parent[x])
++total;
}
prv = x;
x = parent[x];
}
}
int x = m, prv = -1;
bool ok = total <= mid;
int can = 0;
int cnt = 0;
while (ok && x != t) {
++can;
int nw = 0;
for (int to: g[x]) {
if (to != parent[x] && to != prv) {
if (dp[to] + total > mid) {
++cnt;
} else {
++nw;
}
}
}
total -= nw;
if (cnt > can) {
ok = false;
}
prv = x;
x = parent[x];
}
if (ok) {
r = mid;
} else {
l = mid;
}
}
cout << r;
return 0;
}
