#include <bits/stdc++.h>
using namespace std;
vector<vector<int>> tree;
vector<int> path;
bool pth(int v, int p, int x){
if(v == x){
path.push_back(v); return 1;
}
for(int u: tree[v]){
if(u==p) continue;
if(pth(u, v, x)){
path.push_back(v);
return 1;
}
}
return 0;
}
int dfs(int p,int n, int ev)
{
int m=0;
vector<int> v;
for(auto c:tree[n])
{
if(c==ev) continue;
if(c!=p)
{
int a=dfs(n,c, ev);
v.push_back(a);
}
}
sort(v.begin(),v.end());
reverse(v.begin(),v.end());
for(int i=0;i<v.size();i++)
{
m=max(m,v[i]+i+1);
}
return m;
}
int trasferisci(int N, int R, int S, vector<int> U, vector<int> V) {
tree.resize(N);
for(int i=0;i<N-1;i++)
{
tree[U[i]].push_back(V[i]);
tree[V[i]].push_back(U[i]);
}
if(S== R) return dfs(-1,S, -1);
pth(S, -1, R);
//for(int i: path) cout << i << ' '; cout << '\n';
int l = 0; int r = path.size()-2;
while(r-l>1){
int m = (l+r)/2;
int sls = dfs(-1, S, path[m]);
int slr = dfs(-1, R, path[m+1]);
if(sls>slr) l = m;
else if(sls == slr) return sls;
else r = m;
}
return min(max(dfs(-1, S, path[l]), dfs(-1, R,path[l+1])), max(dfs(-1, S, path[r]), dfs(-1, R,path[r+1])));
}
int main () {
int N, R, S;
cin >> N >> R >> S;
vector<int> U(N - 1), V(N - 1);
for (int i = 0; i < N - 1; i++)
cin >> U[i] >> V[i];
cout << trasferisci(N, R, S, U, V) << "\n";
}
