#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));
if (t.empty())
return 0;
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)
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;
}
unsigned max_cover(unsigned s, vector<unsigned> const &path, unsigned t)
{
unsigned a = 0, b = path.size() - 1;
while (a < b)
{
unsigned mid = (a + b + 1) / 2;
if (calc_dp(path[1], s, path[mid]) > t)
b = mid - 1;
else
a = mid;
}
return a;
}
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> path1, path2;
find_path(s2, -1, s1, path1);
find_path(s1, -1, s2, path2);
if (path1.size() == 1)
{
cout << max(calc_dp(s1, s2), calc_dp(s2, s1)) << '\n';
return 0;
}
vector<unsigned> x, y;
for (unsigned v : g[s1])
if (v != path1[0])
x.push_back(calc_dp(v, s1));
for (unsigned v : g[s2])
if (v != path2[0])
y.push_back(calc_dp(v, s2));
sort(x.begin(), x.end(), greater<unsigned>());
sort(y.begin(), y.end(), greater<unsigned>());
unsigned a = 0, b = n;
while (a < b)
{
unsigned t = (a + b) / 2;
unsigned d1 = 0, d2 = 0;
bool possible = 1;
for (size_t i = 0; i < x.size() && possible; i++)
{
if (t < x[i] + i + 1 + (d1 != UINT_MAX))
possible = 0;
if (t <= x[i] && (!i || t >= x[i + 1]))
d1 = i + 1;
}
for (size_t i = 0; i < y.size() && possible; i++)
{
if (t < y[i] + i + 1 + (d2 != UINT_MAX))
possible = 0;
if (t <= y[i] && (!i || t >= y[i + 1]))
d2 = i + 1;
}
if (possible && max_cover(s1, path1, t - d1) >= path2.size() - max_cover(s2, path2, t - d2) - 1)
b = t;
else
a = t + 1;
}
cout << a << '\n';
}
