#include <iostream>
#include <vector>
#include <algorithm>
#define pb push_back
#define all(xs) xs.begin(), xs.end()
#define rep(i, n) for (int i=0; i<(n); i++)
using namespace std;
int N, T, M;
vector<int> G[1000000], V[1000000];
bool sub[1000000];
int R[1000000];
void dfs(int x, int p) {
sub[x] = x == M;
for (int t : G[x]) {
if (t == p) continue;
dfs(t, x);
sub[x] |= sub[t];
}
}
struct Max2 {
int m1 = -1, m2 = -1;
void add(int x) {
if (x > m1) m2 = m1, m1 = x;
else if (x > m2) m2 = x;
}
};
int dfs2(int x, int p) {
Max2 max2;
int ctr = 0;
for (int t : G[x]) {
if (t == p) continue;
max2.add(dfs2(t, x));
ctr++;
}
if (max2.m2 != -1) ctr += max2.m2;
return ctr;
}
vector<int> path;
bool solve(int X) {
//cout<<"solve("<<X<<")\n";
int all = 0, stock = 0;
for (int v : path) all += R[v];
for (int v : path) {
stock++;
//cout<<"all="<<all<<"\n";
bool ok = false;
if (V[v].empty()) ok = all <= X;
else if (all+V[v][0] <= X) ok = true; // do nothing
else {
int ctr = 0;
rep(i, V[v].size()) {
int next = (i+1 == V[v].size())? 0 : V[v][i+1];
if (stock == 0) return false;
//cout<<"take "<<V[v][i]<<" (next="<<next<<"): stock="<<stock<<"--\n";
stock--, ctr++;
if (all+next <= X) {
ok = true;
break;
}
}
all += ctr;
}
if (!ok) return false;
all -= R[v];
}
return all <= X;
}
signed main() {
cin >> N >> T >> M;
T--, M--;
if (T == M) {
cout << 0 << "\n";
return 0;
}
rep(i, N-1) {
int u, v;
cin >> u >> v;
u--, v--;
G[u].pb(v);
G[v].pb(u);
}
dfs(T, -1);
int u = M, par = -1;
while (true) {
path.pb(u);
for (int t : G[u]) if (t != par && sub[t]) {
par = u, u = t;
break;
}
if (u == T) break;
}
for (int i : path) {
for (int t : G[i]) if (!sub[t]) V[i].pb(dfs2(t, i)), R[i]++;
sort(all(V[i]), greater<int>());
}
//for (int x:path){ cout<<"{"; for (int t:V[x])cout<<t<<","; cout<<"},"; }cout<<"\n";
int lo = -1, hi = N+2;
while (hi - lo > 1) {
int mid = (lo + hi) / 2;
if (solve(mid)) hi = mid;
else lo = mid;
}
cout << hi << "\n";
return 0;
}
