#include <bits/stdc++.h>
using namespace std;
const int maxn = 300005;
int n, a, b;
vector<int> g[maxn];
int parent[maxn];
void dfsInit(int node, int par)
{
parent[node] = par;
for (auto to : g[node])
{
if (to != par)
{
dfsInit(to, node);
}
}
}
vector<int> path;
bool marked[maxn];
int dfs(int node, int par, int lastStop)
{
vector<int> childrenDP;
for (auto to : g[node])
{
if (to == par) continue;
if (node == lastStop && marked[to] == true) continue;
childrenDP.push_back(dfs(to, node, lastStop));
}
if (childrenDP.empty()) return 0;
sort(childrenDP.begin(), childrenDP.end());
reverse(childrenDP.begin(), childrenDP.end());
int result = 0;
for (int i = 0; i < childrenDP.size(); ++i)
{
result = max(result, i + 1 + childrenDP[i]);
}
return result;
}
int currentAnswer;
bool check(int node1, int node2)
{
int resultA = dfs(a, -1, node1);
int resultB = dfs(b, -1, node2);
currentAnswer = max(resultA, resultB);
if (resultA < resultB) return true;
else return false;
}
void fastIO()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
}
int main()
{
fastIO();
cin >> n >> a >> b;
for (int i = 1; i < n; ++i)
{
int x, y;
cin >> x >> y;
g[x].push_back(y);
g[y].push_back(x);
}
dfsInit(a, -1);
int curr = b;
while (curr != -1)
{
path.push_back(curr);
marked[curr] = true;
curr = parent[curr];
}
reverse(path.begin(), path.end());
int ans = (1 << 30);
/*for (int i = 0; i < path.size() - 1; ++i)
{
check(path[i], path[i + 1]);
ans = min(ans, currentAnswer);
}
cout << ans << endl;*/
int l = 0;
int r = path.size() - 2;
while (l <= r)
{
int mid = (l + r) / 2;
if (check(path[mid], path[mid + 1]) == true)
{
l = mid + 1;
}
else
{
r = mid - 1;
}
}
if (path.size() == 2)
{
check(path[0], path[1]);
cout << currentAnswer << endl;
}
else
{
int ans = (1 << 30);
if (l + 1 < path.size())
{
check(path[l], path[l + 1]);
ans = min(ans, currentAnswer);
}
if (l >= 1)
{
check(path[l], path[l - 1]);
ans = min(ans, currentAnswer);
}
assert(ans != (1 << 30));
cout << ans << endl;
}
return 0;
}
