#include <bits/stdc++.h>
#define int long long
int n,a,b;
std::vector<int> adj[300100];
std::vector<int> path;
int time(int curr,int parr,int ignore){
std::vector<int> tot;
for(auto to:adj[curr]){
if(to==parr||to==ignore)continue;
tot.push_back(time(to,curr,ignore));
}
std::sort(tot.rbegin(),tot.rend());
int max = 0;
for(int i=0;i<tot.size();i++){
max=std::max(tot[i]+i+1,max);
}
return max;
}
bool solve(int req){
int l=0,r=path.size()-1;
while(l<r){
int mid = (l+r+1)/2;
if(mid==0||time(a,a,path[mid])<=req){
l=mid;
}
else{
r=mid-1;
}
}
if(l==0)return false;
return time(b,b,path[l-1])<=req;
}
bool findPath(int curr,int par){
if(curr==b){
path.push_back(curr);
return true;
}
for(auto to:adj[curr]){
if(to==par)continue;
if(findPath(to,curr)){
path.push_back(curr);
return true;
}
}
return false;
}
signed main() {
std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);
std::cin >> n >> a >> b;
for(int i=0;i<n-1;i++){
int in1,in2;
std::cin >> in1 >> in2;
adj[in1].push_back(in2);
adj[in2].push_back(in1);
}
findPath(a,a);
std::reverse(path.begin(),path.end());
int l=1,r=n;
while(l<r){
int mid = (l+r)/2;
if(solve(mid)){
r=mid;
}
else{
l=mid+1;
}
}
std::cout << r;
}
