// https://oj.uz/problem/view/CEOI17_mousetrap
#include<iostream>
#include<vector>
#include<queue>
#include<algorithm>
#include<functional>
#include<cassert>
int nn,trap,mouse;
std::vector<std::vector<int>> ad;
std::vector<int> par; // with root = trap
std::vector<int> nrock; // num of rock to put if mouse is trapped at [?]
// of course the mouse can only be trapped at nodes with <= 1 child (rooted somewhere)
void dfs1(int x){
int const px=par[x];
nrock[x]=nrock[px]+ad[x].size()-2;
for(int child:ad[x])if(child!=px){
par[child]=x;
dfs1(child);
}
}
std::vector<int> d1; // dist(?, line(trap, mouse))
std::vector<int> cost; // total cost if mouse is trapped at [?] = nrock + d1
int thr;
bool dfs3(int x,int par){ // is it possible to force mouse to be trapped at a node with cost <= thr ...
// when start at x?
int nchild=ad[x].size()-(par>=0);
if(nchild==0)return cost[x]<=thr;
if(nchild==1){
if(cost[x]<=thr)return true;
return false; // ???
}
int n_impos=0;
for(int child:ad[x])if(child!=par){
n_impos+=!dfs3(child,x);
}
return n_impos<=1;
}
int main(){
std::ios::sync_with_stdio(false);std::cin.tie(nullptr);
std::cin>>nn>>trap>>mouse;--trap;--mouse;
ad.resize(nn);
for(int u,v,_=nn-1;_--;){
std::cin>>u>>v;--u;--v;
ad[u].push_back(v);
ad[v].push_back(u);
}
nrock.resize(nn);nrock[trap]=1;
par.resize(nn);par[trap]=-1;
for(int x:ad[trap]){
par[x]=trap;
dfs1(x);
}
d1.resize(nn,-1);
std::queue<int> q;
for(int x=mouse;x>=0;x=par[x]){
q.push(x);
d1[x]=0;
}
while(!q.empty()){
int const x=q.front();q.pop();
int const newd=d1[x]+1;assert(newd>0);
for(int y:ad[x])if(d1[y]<0){
d1[y]=newd;
q.push(y);
}
}
cost.resize(nn);
std::transform(begin(nrock),end(nrock),begin(d1),begin(cost),std::plus<int>{});
thr=nn;
for(int r=1<<29;r;r>>=1){
if(thr-r<0)continue;
thr-=r;
if(!dfs3(mouse,-1))
thr+=r;
}
std::cout<<thr<<'\n';
return 0;
}
/*
10 1 4
1 2
2 3
2 4
3 9
3 5
4 7
4 6
6 8
7 10
*/
