#include <bits/stdc++.h>
using namespace std;
vector<vector<unsigned>> g;
unsigned solve_for_one(unsigned u, unsigned p, vector<unsigned> &dp, unsigned w = -1)
{
if (g[u].size() == 1 && g[u][0] == p)
return 0;
vector<unsigned> children_times;
for (unsigned v : g[u])
if (v != p && v != w)
children_times.push_back(solve_for_one(v, u, dp, w));
sort(children_times.begin(), children_times.end());
unsigned total = 0, i = 1;
for (auto it = children_times.rbegin(); it != children_times.rend(); it++)
{
total = max(*it + i, total);
i++;
}
dp[u] = total;
return total;
}
bool find_path(unsigned u, unsigned p, unsigned v, vector<unsigned> &path)
{
path.push_back(u);
if (u == v)
return 1;
for (unsigned x : g[u])
if (x != p)
if (find_path(x, u, v, path))
return 1;
path.pop_back();
return 0;
}
int main()
{
size_t n;
unsigned a, b;
cin >> n >> a >> b;
a--;
b--;
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);
}
vector<unsigned> p;
find_path(a, -1, b, p);
unsigned l = p.size() - 1;
vector<unsigned> dp1(n), dp2(n);
unsigned x1 = solve_for_one(a, p[1], dp1),
x2 = solve_for_one(b, p[p.size() - 2], dp1),
x3 = solve_for_one(p[1], a, dp1, b) + 1,
x4 = solve_for_one(p[p.size() - 2], b, dp2, a) + 1;
unsigned x5 = UINT_MAX, x6 = UINT_MAX, x7 = UINT_MAX;
for (unsigned i = 0; i < l; i++)
{
vector<unsigned> xtime1, ytime2;
unsigned x = p[i], y = p[i + 1];
for (unsigned v : g[x])
{
if (v != y && (!i || v != p[i - 1]))
{
xtime1.push_back(dp1[v]);
}
}
for (unsigned v : g[y])
{
if (v != x && (i < p.size() - 1 || v != p[i + 1]))
{
ytime2.push_back(dp2[v]);
}
}
sort(xtime1.begin(), xtime1.end());
sort(ytime2.begin(), ytime2.end());
unsigned xv1 = 0, yv2 = 0;
unsigned j = 1;
for (auto it = xtime1.rbegin(); it != xtime1.rend(); it++)
{
xv1 = max(*it + j, xv1);
j++;
}
j = 1;
for (auto it = ytime2.rbegin(); it != ytime2.rend(); it++)
{
yv2 = max(*it + j, yv2);
j++;
}
x5 = min(x5, max(xv1 + i, yv2 + l - i));
x6 = min(x6, max(xv1 + i + 1, yv2 + l - i));
x7 = min(x7, max(xv1 + i, yv2 + l - i + 1));
}
if (p.size() == 2)
x3 = x4 = x5 = x6 = x7 = 0;
unsigned min_cost = UINT_MAX;
min_cost = min(min_cost, max(x1, max(x2, x5 + 1)));
min_cost = min(min_cost, max(x1 + 1, max(x2 + 1, x5)));
min_cost = min(min_cost, max(x1, max(x2 + 1, x6)));
min_cost = min(min_cost, max(x2, max(x1 + 1, x7)));
min_cost = min(min_cost, max(x1, max(x2 + 1, x4)));
min_cost = min(min_cost, max(x1, max(x2, x4 + 1)));
min_cost = min(min_cost, max(x2, max(x1 + 1, x3)));
min_cost = min(min_cost, max(x2, max(x1, x3 + 1)));
cout << min_cost << '\n';
}
