#include <bits/stdc++.h>
using namespace std;
pair <int , int> interzis;
vector <int> adiacenta[300001];
int ordine[300001] , raspuns[300001][2];
inline bool GetPath (const int nod , const int dorit , const int sursa)
{
if (nod == dorit)
{
ordine[++ordine[0]] = nod;
return true;
}
for (auto& vecin : adiacenta[nod]) {
if (vecin != sursa) {
if (GetPath(vecin , dorit , nod))
{
ordine[++ordine[0]] = nod;
return true;
}
}
}
return false;
}
inline int Calcul (const int nod , const int sursa)
{
vector <int> candidati;
for (auto& vecin : adiacenta[nod]) {
if (vecin != sursa && !((nod == interzis.first && vecin == interzis.second) || (nod == interzis.second && vecin == interzis.first)))
{ candidati.push_back(Calcul(vecin , nod) + 1); }
}
sort(candidati.begin() , candidati.end() , greater <int> ());
int maxim = 0;
for (int indice = 0 ; indice < (int)candidati.size() ; indice++)
{ maxim = max(maxim , candidati[indice] + indice); }
return maxim;
}
inline int Search (const int inceput , const int limita , const int tip)
{
int nod = 1 , stanga = 1 , dreapta = ordine[0] - 1;
while (stanga <= dreapta)
{
const int candidat = (stanga + dreapta) >> 1;
if (raspuns[nod][tip] == -1) {
interzis = {ordine[candidat] , ordine[candidat + 1]};
raspuns[nod][tip] = Calcul(inceput , 0);
}
if (raspuns[nod][tip] <= limita) {
if (tip == 0) { nod += candidat - stanga + 1; stanga = candidat + 1; }
else { nod++; dreapta = candidat - 1; }
} else {
if (tip == 0) { nod++; dreapta = candidat - 1; }
else { nod += candidat - stanga + 1; stanga = candidat + 1; }
}
}
return (tip == 0 ? dreapta : stanga);
}
int main ()
{
ios :: sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
int numar_noduri , inceput[2];
cin >> numar_noduri >> inceput[0] >> inceput[1];
for (int indice = 1 ; indice < numar_noduri ; indice++)
{
int nod[2]; cin >> nod[0] >> nod[1];
adiacenta[nod[0]].push_back(nod[1]);
adiacenta[nod[1]].push_back(nod[0]);
}
GetPath(inceput[0] , inceput[1] , 0);
for (int indice = 1 ; indice <= ordine[0] ; indice++)
{ raspuns[indice][0] = raspuns[indice][1] = -1; }
int stanga = 0 , dreapta = numar_noduri;
while (stanga <= dreapta)
{
const int candidat = (stanga + dreapta) >> 1;
const int limita_1 = Search(inceput[0] , candidat , 0);
const int limita_2 = Search(inceput[1] , candidat , 1);
if (limita_1 >= limita_2) { dreapta = candidat - 1; }
else { stanga = candidat + 1; }
}
cout << stanga;
return 0;
}
