#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long
#define ld long double
#define ull unsigned long long
#define ff first
#define ss second
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vl vector<long long>
#define pb push_back
#define rep(i, b) for(int i = 0; i < (b); ++i)
#define rep2(i,a,b) for(int i = a; i <= (b); ++i)
#define rep3(i,a,b,c) for(int i = a; i <= (b); i+=c)
#define count_bits(x) __builtin_popcountll((x))
#define all(x) (x).begin(),(x).end()
#define siz(x) (int)(x).size()
#define forall(it,x) for(auto& it:(x))
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
//mt19937 mt;void random_start(){mt.seed(chrono::time_point_cast<chrono::milliseconds>(chrono::high_resolution_clock::now()).time_since_epoch().count());}
//ll los(ll a, ll b) {return a + (mt() % (b-a+1));}
const int INF = 1e9+50;
const ll INF_L = 1e18+40;
const ll MOD = 1e9+7;
vi graph[1000001];
int tdist[1000001];
int ans[1000001];
int dp[1000001];
int P[1000001];
bool path[1000001];
vi p;
int n,t,m;
void dfs_tdist(int v, int pop, int d)
{
P[v] = pop;
tdist[v] = d++;
forall(it,graph[v])
{
if(it != pop)
{
dfs_tdist(it,v,d);
}
}
}
void dfs_mdist(int v, int pop, int min_dep)
{
min_dep = min(min_dep,tdist[v]);
ans[v] = tdist[v] - min_dep;
forall(it,graph[v])
{
if(it != pop)
{
dfs_mdist(it,v,min_dep);
}
}
}
void dfs_ans(int v, int pop, int cur_add)
{
cur_add += siz(graph[v])-1;
ans[v] += cur_add;
cur_add--;
if(v == t) cur_add = 0;
forall(it,graph[v])
{
if(it != pop)
{
if(!path[v] || path[it] || v == m)
{
dfs_ans(it,v,cur_add);
}
else
{
dfs_ans(it,v,cur_add-1);
}
}
}
}
void dfs_dp(int v, int pop)
{
vi dps;
forall(it,graph[v])
{
if(it != pop)
{
dfs_dp(it,v);
dps.pb(dp[it]);
}
}
sort(all(dps));
reverse(all(dps));
if(siz(dps) >= 2) dp[v] = dps[1];
else if(siz(dps) == 1) dp[v] = min(ans[v]+1,dps[0]);
else dp[v] = ans[v];
}
bool check(int ans2)
{
int cur_moves = 1;
int cur_used = 0;
forall(v,p)
{
int prev_used = cur_used;
forall(it,graph[v])
{
if(path[it]) continue;
if(prev_used + dp[it] > ans2)
{
cur_moves--;
cur_used++;
}
}
if(cur_moves < 0) return 0;
cur_moves++;
}
if(cur_used <= ans2)
{
return 1;
}
return 0;
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
//random_start();
cin >> n >> t >> m;
rep(i,n-1)
{
int a,b;
cin >> a >> b;
graph[a].pb(b);
graph[b].pb(a);
}
dfs_tdist(t,t,0);
dfs_mdist(m,m,tdist[m]);
int cur = m;
while(cur != t)
{
path[cur] = 1;
p.pb(cur);
cur = P[cur];
}
path[t] = 1;
dfs_ans(t,t,0);
forall(it,p)
{
forall(it2,graph[it])
{
if(path[it2]) continue;
dfs_dp(it2,it);
}
}
int l = 0;
int r = n*2;
int ans = -1;
while(l <= r)
{
int mid = (l+r)/2;
if(check(mid))
{
ans = mid;
r = mid-1;
}
else
{
l = mid+1;
}
}
cout << ans << "\n";
}
