#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
const int MAXN = 1e6+10;
int n, st, e, p[MAXN], pid[MAXN], dp[MAXN], ans[MAXN], sz[MAXN], cnt[MAXN], ans2[MAXN], bns2[MAXN];
bool mark[MAXN], stuck[MAXN];
vector <pair<int,int>> adj[MAXN];
vector <pair<int,int>> ch[MAXN];
inline void dfs(int v, int parv){
sz[v] = 1;
for(auto i: adj[v]){
if(i.first==parv) continue;
p[i.first] = v;
pid[i.first] = i.second;
dfs(i.first, v);
sz[v] += sz[i.first];
}
return;
}
inline void dfs1(int v, int parv){
for(auto i: adj[v]){
int u = i.first, id = i.second;
if(u==parv) continue;
if(!mark[id]) cnt[v]++;
dfs1(u,v);
ch[v].push_back({dp[u],u});
if(mark[id]) ans[v] += ans[u];
}
if(cnt[v]==0){
dp[v] = 0; return;
}
int mx1 = 0, mx2 = 0;
for(auto i: adj[v]){
int u = i.first, id = i.second;
if(u==parv || mark[id]) continue;
if(dp[u]>mx1) mx2 = mx1, mx1 = dp[u];
else if(dp[u]>mx2) mx2 = dp[u];
}
dp[v] = 1+mx2+(cnt[v]>1?cnt[v]-1:0); ans[v]+=dp[v];
return;
}
int main()
{
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> n >> st >> e; st--; e--;
for(int i = 0; i < n-1; i++){
int u,v;
cin >> u >> v; u--; v--;
adj[u].push_back({v,i});
adj[v].push_back({u,i});
}
dfs(st,-1);
int u = e;
while(u!=st){
mark[pid[u]] = true;
u = p[u];
}
dfs1(st,-1);
cout << ans[st] << endl;
return 0;
}
