#include <bits/stdc++.h>
using namespace std;
#define task "code"
const int ar = 3e5 + 1;
const int inf = 1e9 + 5;
int n, a, b, u, v, dp[ar], res = inf;
vector<int> adj[ar];
vector< pair<int, int> > vec;
bool c = false;
void find_dfs(int u, int p) {
if (u == b) {
c = true;
return;
}
for (int v : adj[u]) if (v != p) {
find_dfs(v, u);
if (c == true) {
vec.push_back({u, v});
return ;
}
}
}
int dfs (int u, int p, int x) {
vector<int> vc;
for (int v : adj[u])
if (v != p && v != x)
vc.push_back(dfs(v, u, x));
int ans = 0;
sort (vc.begin(), vc.end(), greater<int>());
for (int i = 0; i < vc.size(); ++ i)
ans = max(ans, vc[i] + i + 1);
// cout << u << ' ' << p << ' ' << ans << '\n';
// cout << "vc: ";
// for (int x : vc) cout << x << ' '; cout << '\n';
return ans;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin >> n >> a >> b;
for (int i = 1; i < n; ++ i) {
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
if (a == b) return cout << dfs(a, 0, 0), 0;
find_dfs(a, 0);
int l = 0, r = vec.size() - 1;
while (l <= r) {
int m = l + r >> 1;
// cout << vec[m].first << ' ' << vec[m].second << '\n';
int valA = dfs(a, 0, vec[m].second), valB = dfs(b, 0, vec[m].first);
res = min(res, max(valA, valB));
if (valA >= valB) r = m - 1;
else l = m + 1;
}
cout << res;
return 0;
}
