#include <bits/stdc++.h>
using namespace std;
#define FOR(i, a, b) for(int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define TRACE(x) cerr << #x << " = " << x << endl
#define _ << " " <<
typedef long long ll;
typedef pair<int, int> point;
const int MAXN = 3e5 + 5;
struct edge{
int to, id;
edge(){};
edge(int _to, int _id){
to = _to; id = _id;
}
};
vector <edge> E[MAXN];
vector <int> path, tmp;
int n, a, b;
void load(){
ios_base::sync_with_stdio(false); cin.tie(0);
cin >> n >> a >> b;
REP(i, n - 1){
int x, y; cin >> x >> y;
E[x].push_back(edge(y, i));
E[y].push_back(edge(x, i));
}
}
void findPath(int x, int par = 0){
if(path.size())
return;
if(x == b){
path = tmp;
return;
}
for(auto it : E[x]){
if(it.to == par) continue;
tmp.push_back(it.id);
findPath(it.to, x);
tmp.pop_back();
}
}
int cutEdge;
int calc(int x, int par = 0){
int ret = 0;
vector <int> v;
for(auto it : E[x]){
if(it.to == par) continue;
if(it.id == cutEdge) continue;
v.push_back(calc(it.to, x));
}
sort(v.rbegin(), v.rend());
REP(i, (int)v.size())
ret = max(ret, v[i] + i + 1);
return ret;
}
int main(){
load();
findPath(a);
int lo = 0, hi = path.size() - 1;
while(lo < hi){
int mid = (lo + hi) >> 1;
cutEdge = path[mid];
int left = calc(a);
int right = calc(b);
if(left < right)
lo = mid + 1;
else
hi = mid;
}
cutEdge = path[lo];
int sol = max(calc(a), calc(b));
if(lo - 1 >= 0){
cutEdge = path[lo - 1];
sol = min(sol, max(calc(a), calc(b)));
}
cout << sol;
}
