#include<bits/stdc++.h>
using namespace std;
void DBG() { cerr << "]\n"; }
template<class H, class... T> void DBG(H h, T... t) {
cerr << h; if(sizeof...(t)) cerr << ", ";
DBG(t...);
}
#ifdef LOCAL
#define dbg(...) cerr << "[" << #__VA_ARGS__ << "]: [", DBG(__VA_ARGS__)
#else
#define dbg(...) 0
#endif // LOCAL
#define FOR(i,a,b) for(int i = (a) ; i<(b) ; i++)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for(int i = (b)-1 ; i>=(a) ; i--)
#define R0F(i,a) ROF(i,0,a)
#define each(e,a) for(auto &e : (a))
#define sz(v) (int)(v).size()
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define pb push_back
#define tcT template<class T
#define nl "\n"
using ll = long long;
using vi = vector<int>;
using pi = pair<int,int>;
using str = string;
tcT> using V = vector<T>;
tcT> using pqg = priority_queue<T,vector<T>,greater<T>>;
void setIO(string NAME = "") {
cin.tie(0)->sync_with_stdio(0);
if(sz(NAME)) {
freopen((NAME + ".inp").c_str(),"r",stdin);
freopen((NAME + ".out").c_str(),"w",stdout);
}
}
tcT> bool ckmin(T&a, const T&b) {
return b < a ? a=b,1 : 0; }
tcT> bool ckmax(T&a, const T&b) {
return b > a ? a=b,1 : 0; }
const int MOD = 1e9 + 7;
const int MX = 3e5+5;
int N, A, B;
vi adj[MX];
vi tmp, pathAB;
int dp[MX];
void dfs(int u, int p) {
tmp.pb(u);
if(u == B) {
pathAB = tmp;
return;
}
each(v, adj[u]) if(v != p) {
dfs(v, u);
}
tmp.pop_back();
}
int calc(int u, int p, int blk) {
dp[u] = 0;
if(u == blk) return dp[u];
vi vals;
each(v, adj[u]) if(v != p && v != blk) {
vals.pb(calc(v, u, blk));
}
sort(rall(vals));
F0R(i, sz(vals)) ckmax(dp[u], vals[i] + i + 1);
return dp[u];
}
void solve() {
cin >> N >> A >> B;
F0R(i, N-1) {
int u, v;
cin >> u >> v;
adj[u].pb(v);
adj[v].pb(u);
}
dfs(A, -1);
int lo = 0, hi = sz(pathAB) - 2;
while(lo < hi) {
int mid = lo + (hi - lo + 1)/2;
int resA = calc(A, -1, pathAB[mid+1]);
int resB = calc(B, -1, pathAB[mid]);
if(resA <= resB) lo = mid;
else hi = mid-1;
}
int ans = max(calc(A, -1, pathAB[lo+1]), calc(B, -1, pathAB[lo]));
if(lo + 2 < sz(pathAB)) ckmin(ans, max(calc(B, -1, pathAB[lo+1]), calc(A, -1, pathAB[lo+2])));
cout << ans << nl;
}
int main() {
setIO();
int t=1;
//cin>>t;
while(t-->0) {
solve();
}
return 0;
}
