#include <bits/stdc++.h>
using namespace std;
const int mn = 1005;
vector<int> adj[mn];
list<int> path;
int n, a, b, u, v;
bool find(int x, int p = -1) {
if (x == b) {
path.push_front(b);
return true;
}
for (int i : adj[x]) {
if (i != p && find(i, x)) {
path.push_front(x);
return true;
}
}
return false;
}
int dfs(int x, int p = -1) {
vector<int> s;
int c = 0;
for (int i : adj[x]) {
if ((i == u && x == v) || (i == v && x == u) || i == p)
continue;
c++;
s.push_back(dfs(i, x));
}
sort(s.begin(), s.end(), greater<>());
int ans = 0;
for (int i = 0, l = s.size(); i < l; i++)
ans = max(ans, s[i] + i);
return ans + 1;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
cin >> n >> a >> b;
a--, b--;
for (int i = 0; i < n - 1; i++) {
int x, y;
cin >> x >> y;
x--, y--;
adj[x].push_back(y);
adj[y].push_back(x);
}
find(a);
auto it = path.begin();
u = *it;
int ans = 1e9;
for (it++; it != path.end(); it++) {
v = *it;
ans = min(ans, max(dfs(b) - 1, dfs(a) - 1));
u = v;
}
cout << ans;
}
