#include <bits/stdc++.h>
#define lsb(x) (x & (-x))
#define ll long long
#define ull unsigned long long
/*const int MOD = ;
inline int lgput(int a, int b) {
int ans = 1;
while(b > 0) {
if(b & 1) ans = (1LL * ans * a) % MOD;
b >>= 1;
a = (1LL * a * a) % MOD;
}
return ans;
}
inline void mod(int &x) {
if(x >= MOD)
x -= MOD;
}
inline void add(int &x, int y) {
x += y;
mod(x);
}
inline void sub(int &x, int y) {
x += MOD - y;
mod(x);
}
inline void mul(int &x, int y) {
x = (1LL * x * y) % MOD;
}
inline int inv(int x) {
return lgput(x, MOD - 2);
}*/
/*int fact[], invfact[];
inline void prec(int n) {
fact[0] = 1;
for(int i = 1; i <= n; i++) {
fact[i] = (1LL * fact[i - 1] * i) % MOD;
}
invfact[n] = lgput(fact[n], MOD - 2);
for(int i = n - 1; i >= 0; i--) {
invfact[i] = (1LL * invfact[i + 1] * (i + 1)) % MOD;
}
}
inline int comb(int n, int k) {
if(n < k) return 0;
return (1LL * fact[n] * (1LL * invfact[k] * invfact[n - k] % MOD)) % MOD;
}*/
using namespace std;
const int MAXN = (int) 1e6;
vector <int> g[MAXN + 1];
int father[MAXN + 1];
inline void combine(pair <int, int> &mx, int cur) {
if(mx.first < cur) {
mx.second = mx.first;
mx.first = cur;
}
else if(mx.second < cur) {
mx.second = cur;
}
}
int dp[MAXN + 1];
void go(int nod, int par) {
father[nod] = par;
if(par != 0) {
g[nod].erase(find(g[nod].begin(), g[nod].end(), par));
}
pair <int, int> mx = {0, 0};
for(auto it : g[nod]) {
go(it, nod);
combine(mx, dp[it]);
}
dp[nod] = mx.second + g[nod].size();
}
int main() {
#if 0
ifstream cin("A.in");
ofstream cout("A.out");
#endif
int i, n, t, m;
ios::sync_with_stdio(false);
cin.tie(0), cout.tie(0);
cin >> n >> t >> m;
if(m == t) {
cout << 0;
return 0;
}
for(i = 1; i < n; i++) {
int x, y;
cin >> x >> y;
g[x].push_back(y);
g[y].push_back(x);
}
go(t, 0);
vector <int> anc, sp;
int nod = m;
while(nod != t) {
anc.push_back(nod);
sp.push_back(0);
nod = father[nod];
}
sp.push_back(0);
for(i = (int) sp.size() - 2; i >= 0; i--) {
sp[i] = sp[i + 1] + (int) g[anc[i]].size() - (i > 0);
}
auto check = [&](int X) -> bool {
int cnt = 0;
for(i = 0; i < anc.size(); i++) {
int last = (i == 0 ? 0 : anc[i - 1]), num = 0;
for(auto it : g[anc[i]]) {
if(last == it) continue;
if(cnt + dp[it] + sp[i] - num > X) {
cnt++, num++;
}
if(cnt > i + 1) {
return 0;
}
}
}
return cnt <= X;
};
int res = -1;
for(int step = 1 << 20; step; step >>= 1) {
if(check(res + step) == 0) {
res += step;
}
}
cout << res + 1;
return 0;
}
