#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
const int MOD = (int)1e9 + 7;
const int MAXN = (int)1e6 + 7;
const int infint = (int)1e9;
//try for 25 pts.
int n, t, m, dp[MAXN];
vector<int> G[MAXN];
void dfs(int u, int p)
{
int childs = 0;
for (auto v : G[u])
if(v != p)
childs++;
if(childs == 0)
{
dp[u] = 0;
return;
}
int fir = 0, sec = 0;
for (auto v : G[u])
{
if(v == p)
continue;
dfs(v, u);
if(dp[v] > fir)
sec = fir, fir = dp[v];
else
if(dp[v] > sec)
sec = dp[v];
}
dp[u] = sec + childs;
return;
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
cin >> n >> t >> m;
for (int i = 0; i < n - 1; i++)
{
int u, v;
cin >> u >> v;
G[u].push_back(v);
G[v].push_back(u);
}
dfs(m, t);
cout << dp[m];
}
