#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define all(v) v.begin(), v.end()
#define len(v) (int)(v.size())
#define ff first
#define ss second
typedef pair<int, int> ii;
typedef vector<int> vi;
typedef vector<ii> vii;
const int maxn = 3e5 + 7;
int n, a, b, nm, p[maxn], pi[maxn], ti[maxn];
vi put, t[maxn];
vii ls[maxn];
void dfs1(int v){
for (ii s : ls[v]){
if (s.ff == p[v]) continue;
p[s.ff] = v;
pi[s.ff] = s.ss;
dfs1(s.ff);
}
}
int dfs2(int v, int pa = -1){
t[v].clear();
for (ii s : ls[v]){
if (s.ff == pa || s.ss == nm) continue;
t[v].pb(dfs2(s.ff, v));
}
sort(all(t[v]));
int ret = 0;
for (int i = len(t[v]) - 1; i >= 0; i--)
ret = max(ret, t[v][i] + len(t[v]) - i);
return ret;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin >> n >> a >> b;
for (int i = 1; i < n; i++){
ti[i] = -1;
int u, v;
cin >> u >> v;
ls[u].pb({v, i});
ls[v].pb({u, i});
}
dfs1(a);
int tr = b;
while(tr != a){
put.pb(pi[tr]);
tr = p[tr];
}
reverse(all(put));
int l = 0, r = n;
while(l < r){
int m = (l + r) / 2;
int le = 0, ri = put.size() - 1;
while(le < ri){
int mi = (le + ri + 1) / 2;
nm = put[mi];
int tm;
if (ti[nm] != -1) tm = ti[nm];
else tm = dfs2(a);
ti[nm] = tm;
if (tm <= m) le = mi;
else ri = mi - 1;
}
nm = put[le];
int t1 = dfs2(a), t2 = dfs2(b);
if (max(t1, t2) > m) l = m + 1;
else r = m;
}
cout << l << "\n";
return 0;
}
