#include <bits/stdc++.h>
#define all(x) (x).begin(), (x).end()
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<ull, ull> pull;
typedef pair<int, int> pii;
typedef pair<ld, ld> pld;
int n, t, m;
vector<int> g[1000009];
int trap[1000009];
int dp[1000009];
int edgecnt[1000009];
void dfs(int v, int prt){
if(v == t){
trap[v] = 1;
return;
}
for(auto u : g[v])
if(u != prt){
dfs(u, v);
if(trap[u])
trap[v] = 1;
else
++edgecnt[v];
}
dp[v] = edgecnt[v];
for(auto u : g[v])
if(u != prt && trap[u])
dp[v] += dp[u];
}
int getans(int v, int prt, int curval){
if(v == t) return 0;
if(trap[v] == 0)
curval -= edgecnt[v];
if(curval < 0) return 1e9;
vector<int> vec;
for(auto u : g[v])
if(u != prt && trap[u] == 0)
vec.pb(getans(u, v, curval));
int othneed = 0;
for(auto u : g[v])
if(u != prt && trap[u])
othneed = getans(u, v, curval);
sort(all(vec), greater<int>());
int ret = 1e9;
for(int i = 0; i < vec.size(); ++i)
ret = min(ret, i+max(vec[i], othneed));
ret = min(ret, (int)vec.size()+othneed);
return max(ret-1, 0);
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
//freopen("in.txt", "r", stdin);
//freopen("out.txt", "w", stdout);
cin >> n >> t >> m;
for(int i = 0; i < n-1; ++i){
int x, y;
cin >> x >> y;
g[x].pb(y);
g[y].pb(x);
}
dfs(m, 0);
int l = 0, r = 1e9;
while(l < r){
int mid = (l+r)/2;
if(getans(m, 0, mid-dp[m]) == 0)
r = mid;
else
l = mid+1;
}
cout << l << '\n';
}
