#include <bits/stdc++.h>
#define pb push_back
using namespace std;
struct node_t {
int par;
vector<int> childs;
};
int n, a, b;
vector<vector<int>> adj;
vector<int> path;
vector<node_t> nodes;
class Solution {
public:
struct node_t {
int par, reqt;
vector<int> childs;
};
int ru, rv;
vector<node_t> nodes;
void rootTree(int cur, int par) {
node_t& u = nodes[cur];
if (par != -1) u.par = par;
for (int v : adj[cur]) {
if (v == par or (cur == ru and v == rv) or (cur == rv and v == ru)) continue;
u.childs.pb(v);
rootTree(v, cur);
}
}
void dfs(int cur) {
node_t& u = nodes[cur];
priority_queue<int> pq;
for (int v : u.childs) {
dfs(v);
pq.push(nodes[v].reqt);
}
int t = 1;
while (not pq.empty()) {
u.reqt = max(u.reqt, pq.top() + t);
pq.pop();
t++;
}
}
int ans() {
return max(nodes[a].reqt, nodes[b].reqt);
}
Solution(int ru, int rv) { // restricted vertices
this -> ru = ru;
this -> rv = rv;
nodes.resize(n + 1);
rootTree(a, -1);
rootTree(b, -1);
dfs(a);
dfs(b);
}
};
void rootTree(int cur, int par) {
node_t& u = nodes[cur];
if (par != -1) u.par = par;
for (int v : adj[cur]) {
if (v == par) continue;
u.childs.pb(v);
rootTree(v, cur);
}
}
void findPath(int cur) {
path.pb(cur);
if (cur == a) return;
findPath(nodes[cur].par);
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> a >> b;
adj.resize(n + 1);
nodes.resize(n + 1);
int tu, tv, i;
for (i = 0; i < n - 1; i++) {
cin >> tu >> tv;
adj[tu].pb(tv);
adj[tv].pb(tu);
}
rootTree(a, -1);
findPath(b);
int best = INT_MAX;
for (int i = 0; i < path.size() - 1; i++) {
Solution sol(path[i], path[i + 1]);
best = min(best, sol.ans());
}
cout << best;
// int l = 0, r = path.size() - 2, mid;
// while (l < r - 1) {
// mid = (l + r) / 2;
// Solution left(path[mid], path[mid + 1]), right(path[mid + 1], path[mid + 2]);
// if (left.ans() > right.ans()) l = mid;
// else r = mid;
// }
// Solution left(path[l], path[l + 1]), right(path[l + 1], path[l + 2]);
// if (left.ans() > right.ans()) cout << right.ans();
// else cout << left.ans();
return 0;
}
