#include <bits/stdc++.h>
using namespace std;
vector<vector<unsigned>> g;
vector<unsigned> dp;
unsigned calc_dp(unsigned u, unsigned p = -1, unsigned f = -1)
{
dp[u] = 0;
vector<unsigned> t;
for (unsigned v : g[u])
if (v != p && v != f)
t.push_back(calc_dp(v, u, f));
sort(t.begin(), t.end(), greater<unsigned>());
for (unsigned i = 0; i < t.size(); i++)
dp[u] = max(dp[u], t[i] + i + 1);
return dp[u];
}
bool find_path(unsigned u, unsigned p, unsigned t, vector<unsigned> &path)
{
if (u == t)
{
path.push_back(u);
return 1;
}
for (unsigned v : g[u])
if (v != p)
if (find_path(v, u, t, path))
{
path.push_back(u);
return 1;
}
return 0;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
size_t n;
unsigned s1, s2;
cin >> n >> s1 >> s2;
s1--;
s2--;
g = vector<vector<unsigned>>(n);
for (size_t i = 0; i < n - 1; i++)
{
unsigned u, v;
cin >> u >> v;
g[u - 1].push_back(v - 1);
g[v - 1].push_back(u - 1);
}
dp = vector<unsigned>(n);
vector<unsigned> path;
find_path(s2, -1, s1, path);
if (path.size() == 2)
{
cout << max(calc_dp(s1, s2), calc_dp(s2, s1)) << '\n';
return 0;
}
unsigned a = 0, b = path.size() - 2, min_time = UINT_MAX;
while (a < b)
{
unsigned mid = (a + b) / 2;
unsigned x = calc_dp(s1, -1, path[mid + 1]),
y = calc_dp(s2, -1, path[mid]);
min_time = min(min_time, max(x, y));
if (x < y)
a = mid + 1;
else
b = mid;
}
if (a)
min_time = min(min_time, max(calc_dp(s1, -1, path[a]), calc_dp(s2, -1, path[a - 1])));
cout << min_time << '\n';
}
