#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "C:\GCC\debug.h"
#else
#define debug(...) void(42)
#endif
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n, a, b;
cin >> n >> a >> b;
--a, --b;
vector<vector<array<int, 2>>> adj(n);
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});
}
vector<int> dp(n);
function<void(int, int, int)> Dfs = [&](int u, int prev, int blocked_edge) {
dp[u] = 0;
vector<int> children;
for (auto v : adj[u]) {
if (v[0] == prev || v[1] == blocked_edge) {
continue;
}
Dfs(v[0], u, blocked_edge);
children.push_back(dp[v[0]]);
}
sort(children.begin(), children.end());
for (int i = (int) children.size() - 1; i >= 0; i--) {
int add = (int) children.size() - i;
dp[u] = max(dp[u], children[i] + add);
}
return ;
};
vector<int> edges;
function<bool(int, int)> Root = [&](int u, int prev) {
if (u == b) {
return true;
}
bool found = false;
for (auto v : adj[u]) {
if (v[0] == prev) {
continue;
}
if (Root(v[0], u)) {
found = true;
edges.push_back(v[1]);
}
}
return found;
};
Root(a, -1);
int res = 12345678;
reverse(edges.begin(), edges.end());
int low = 0, high = (int) edges.size() - 1;
while (low <= high) {
int mid = low + (high - low) / 2;
Dfs(a, -1, edges[mid]);
Dfs(b, -1, edges[mid]);
if (dp[a] <= dp[b]) {
low = mid + 1;
} else {
high = mid - 1;
}
}
--low;
if (low >= 0) {
Dfs(a, -1, edges[low]);
Dfs(b, -1, edges[low]);
res = min(res, max(dp[a], dp[b]));
}
++low;
if (low < (int) edges.size()) {
Dfs(a, -1, edges[low]);
Dfs(b, -1, edges[low]);
res = min(res, max(dp[a], dp[b]));
}
cout << res << '\n';
return 0;
}
