#include<bits/stdc++.h>
using namespace std;
const int N = 3e5+5;
vector<int> G[N], child[N];
vector<int> pth;
bool dead[N];
int mx[N], cnt[N], mi[N];
bool sorter(int a, int b)
{
return mx[a] < mx[b];
}
void path(int s, int d, int p = 0)
{
pth.push_back(s);
for(int u : G[s])
if(u != p && pth.back() != d)
path(u, d, s);
if(pth.back() != d)
pth.pop_back();
}
void dfs_child(int v, int p = 0)
{
for(int u : G[v])
if(!dead[u] && p != u)
{
child[v].push_back(u);
dfs_child(u, v);
}
}
void dfs_dp(int v)
{
// cerr << v << ' ';
for(int c : child[v])
dfs_dp(c);
sort(child[v].rbegin(), child[v].rend(), sorter);
vector<int> vec;
int i = 1;
for(int c : child[v])
vec.push_back(mx[c] + i), i++;
if(vec.empty())
{
mx[v] = mi[v] = cnt[v] = 0;
return;
}
sort(vec.begin(), vec.end());
// cerr << "V = " << v << endl;
mx[v] = vec.back();
mi[v] = vec.front();
for(int i : vec)
cnt[v] += (i == vec.back());
}
int main()
{
int n, a, b;
cin >> n >> a >> b;
for(int i = 1; i < n; i ++)
{
int u, v;
cin >> u >> v;
G[u].push_back(v);
G[v].push_back(u);
}
path(a, b);
for(int v : pth)
dead[v] = true;
for(int v : pth)
dfs_child(v);
for(int v : pth)
{
//cerr << "Tree : ";
dfs_dp(v);
//cerr << endl;
// cerr << "mx[" << v << "] = " << mx[v] << endl;
// cerr << "cnt[" << v << "] = " << cnt[v] << endl;
}
// for(int v : pth)
// cout << v << ' ' << cnt[v] << '\n';
int l = -1, r = n + 1;
while(r - l > 1)
{
int t = (l + r) / 2;
bool can = true;
// for(int v : pth)
// cout << cnt[v] << ' ';
//cout << endl;
int tm[pth.size()];
int add = 0, i = 0;
for(int v : pth)
{
if(add + mx[v] < t)
tm[i] = mx[v] + add + 1, add++;
else
tm[i] = mx[v] + add, add += cnt[v];
i++;
}
add = 0;
for(i = pth.size() - 1; i >= 0; i--)
{
int v = pth[i];
// cerr << add << ' ' << v << endl;
if(add + mx[v] < t)
tm[i] = min(tm[i], mx[v] + add + 1), add += 1;
else
tm[i] = min(tm[i], mx[v] + add), add += cnt[v];
}
for(int i = 0; i < pth.size(); i++)
if(tm[i] > t)
can = false;
if(can)
r = t;
else
l = t;
}
cout << r << endl;
return 0;
}
