#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define fi first
#define se second
#define mp make_pair
#define fastIO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
const int N = (int)1e6 + 100;
vector<int> T[N];
vector<pii> lis[N];
int dp[N];
int up[N];
int pp[N];
int trap;
int mouse;
void dfs(int u, int par){
int deg = 0;
for(auto x : T[u]) if(x != par) deg ++ ;
for(auto x : T[u]){
if(x==par) continue;
pp[x]=u;
up[x] += up[u] + (par != -1) * (deg - 1);
dfs(x,u);
lis[u].push_back(mp(dp[x],x));
}
sort(lis[u].begin(), lis[u].end());
int mx = 0;
if(lis[u].size() >= 2)
mx = lis[u][lis[u].size() - 2].fi;
dp[u]=mx+lis[u].size();
reverse(lis[u].begin(), lis[u].end());
}
bool can(int mid){
int cur = mouse;
int can = 1;
int alr = 0;
int pv = -1;
int cnt;
int need;
while(cur != trap){
cnt = 0;
for(auto x : lis[cur]){
if(x.se == pv) continue;
cnt ++ ;
}
need = 0;
for(auto x : lis[cur]){
if(x.se == pv) continue;
if(up[cur]+x.fi+cnt+alr > mid){
need ++ ;
if(can == 0){
return false;
}
can -- ;
}
}
alr += need;
pv=cur;
cur = pp[cur];
can++;
}
if(alr > mid) return false;
return true;
}
int main(){
fastIO;
int n;
cin >> n;
cin >> trap >> mouse;
int u, v;
for(int i = 1; i < n; i ++ ){
cin >> u >> v;
T[u].push_back(v);
T[v].push_back(u);
}
dfs(trap,-1);
int lf = 0, rf = (int)n * 2;
int mid;
while(lf < rf){
mid = (lf + rf) / 2;
if(can(mid)){
rf=mid;
}
else{
lf=mid+1;
}
}
cout << lf << "\n";
return 0;
}
