#include <bits/stdc++.h>
#define MEM 300005
#define sanic ios_base::sync_with_stdio(0)
#define x first
#define y second
#define pb push_back
#define all(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pi;
const ll MOD = 1e9+7;
const ll INF = 1e10+7;
ll t,n,m,a,b,ce;
ll v[MEM];
vector<pi> g[MEM];
vector<ll> d,f;
void dfs(ll s, ll e){
v[s] = 1;
if(s==b) {
f = d;
return;
}
for(pi nt: g[s]){
if(v[nt.x] || e==nt.x) continue;
d.pb(nt.y);
dfs(nt.x,s);
d.pop_back();
}
}
ll DP(ll c, ll p){
vector<ll> dp;
for(pi sb: g[c]){
if(p==sb.x || sb.y==ce) continue;
dp.pb(-DP(sb.x,c));
}
sort(all(dp));
ll ans=0;
for(int i=0; i<dp.size(); i++)
ans = max(ans, -dp[i]+i+1);
return ans;
}
main()
{
sanic; cin.tie(0); cout.tie(0);
cin >> n >> a >> b;
for(int i=1; i<n; i++){
ll q1,q2;
cin >> q1 >> q2;
g[q1].pb({q2,i});
g[q2].pb({q1,i});
}
dfs(a,-1);
ll l=0, r=f.size()-1,res=0;
while(l<=r){
ll mid = (l+r)>>1;
ce = f[mid];
if(DP(a,-1)<DP(b,-1)) l = mid+1, res = mid;
else r = mid-1;
}
ce = f[res];
ll ans=max(DP(a,-1),DP(b,-1));
if(res<f.size()-1){
res++;
ce = f[res];
ans = min(ans, max(DP(a,-1),DP(b,-1)));
}
cout << ans;
}
