#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define FOR(i, a, b) for(int i = (a), _b = (b); i <= _b; ++i)
#define REP(i, a, b) for(int i = (a), _b = (b); i >= _b; --i)
#define mp make_pair
#define all(v) v.begin(), v.end()
#define uni(v) v.erase(unique(all(v)), v.end())
#define Bit(x, i) ((x >> (i)) & 1)
#define Mask(i) (1 << (i))
#define Cnt(x) __builtin_popcount(x)
#define Cntll(x) __builtin_popcountll(x)
#define Ctz(x) __builtin_ctz(x)
#define Ctzll(x) __builtin_ctzll(x)
#define Clz(x) __builtin_clz(x)
#define Clzll(x) __builtin_clzll(x)
inline bool maximize(int &u, int v){ return v > u ? u = v, true : false; }
inline bool minimize(int &u, int v){ return v < u ? u = v, true : false; }
inline bool maximizell(long long &u, long long v){ return v > u ? u = v, true : false; }
inline bool minimizell(long long &u, long long v){ return v < u ? u = v, true : false; }
const int mod = 1e9 + 7;
inline int fastPow(int a, int n){
if(n == 0) return 1;
int t = fastPow(a, n >> 1);
t = 1ll * t * t % mod;
if(n & 1) t = 1ll * t * a % mod;
return t;
}
inline void add(int &u, int v){ u += v; if(u >= mod) u -= mod; }
inline void sub(int &u, int v){ u -= v; if(u < 0) u += mod; }
const int maxN = 3e5 + 5;
const int inf = 1e9;
const long long infll = 1e18;
int n;
vector<int> adj[maxN];
int cen1, cen2, f[maxN];
int p[maxN], d[maxN];
bool cmp(const int &a, const int &b){
return f[a] > f[b];
}
void pre(int u){
for(int v : adj[u]){
if(v == p[u]) continue;
p[v] = u;
d[v] = d[u] + 1;
pre(v);
}
}
vector<int> path(int u, int v){
vector<int> node1, node2;
while(u != v){
if(d[u] < d[v]){
node2.emplace_back(v);
v = p[v];
}else{
node1.emplace_back(u);
u = p[u];
}
}
node1.emplace_back(u);
reverse(all(node2));
for(int u : node2)node1.emplace_back(u);
return node1;
}
int ban1, ban2;
void dfs1(int u, int p){
for(int v : adj[u]){
if(v == p || v == ban2) continue;
dfs1(v, u);
}
f[u] = 0;
sort(all(adj[u]), cmp);
int cnt = 1;
for(int v : adj[u]){
if(v == p || v == ban2) continue;
maximize(f[u], f[v] + cnt);
++cnt;
}
}
void dfs2(int u, int p){
for(int v : adj[u]){
if(v == p || v == ban1) continue;
dfs2(v, u);
}
f[u] = 0;
sort(all(adj[u]), cmp);
int cnt = 1;
for(int v : adj[u]){
if(v == p || v == ban1) continue;
maximize(f[u], f[v] + cnt);
++cnt;
}
}
void solve(){
pre(1);
vector<int> node = path(cen1, cen2);
int l = 0, r = node.size() - 2;
int ans = inf;
while(l <= r){
int mid = (l + r) >> 1;
ban1 = node[mid];
ban2 = node[mid + 1];
dfs1(cen1, 0);
dfs2(cen2, 0);
// cout << f[cen1] << ' ' << f[cen2] << '\n';
// cout << l << ' '
if(f[cen1] == f[cen2]){
cout << f[cen1];
return;
}
if(f[cen1] < f[cen2]){
l = mid + 1;
minimize(ans, f[cen2]);
}else{
r = mid - 1;
minimize(ans, f[cen1]);
}
}
cout << ans;
}
void process(){
cin >> n >> cen1 >> cen2;
FOR(i, 1, n - 1){
int u, v;
cin >> u >> v;
adj[u].emplace_back(v);
adj[v].emplace_back(u);
}
solve();
}
#define LOVE "torrent"
int main(){
if(fopen(LOVE".inp", "r")){
freopen(LOVE".inp", "r", stdin);
freopen(LOVE".out", "w", stdout);
}
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int t = 1;
// cin >> t;
while(t--)
process();
// cerr << '\n' << "Time elapsed: " << (1.0 * clock() / CLOCKS_PER_SEC) << " s\n" ;
return 0;
}
