#include <bits/stdc++.h>
using namespace std;
const int NMAX = 1000010;
vector <int> adia[NMAX];
int cost[NMAX];
int tata[NMAX];
int nr_op;
void dfs(int nod, int t)
{
if (t)
adia[nod].erase(find(adia[nod].begin(), adia[nod].end(), t));
tata[nod] = t;
for (auto i : adia[nod])
dfs(i, nod);
int max1 = 0, max2 = 0;
for (auto i : adia[nod]) {
if (cost[i] > max1)
max2 = max1, max1 = cost[i];
else if (cost[i] > max2)
max2 = cost[i];
}
cost[nod] = adia[nod].size() + max2;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, root, x;
cin >> n >> root >> x;
if (root == x) {
cout << "0\n";
return 0;
}
for (int i = 1; i < n; i++) {
int a, b;
cin >> a >> b;
adia[a].push_back(b);
adia[b].push_back(a);
}
dfs(root, 0);
int ans = cost[x];
x = tata[x];
while (x != root) {
ans += adia[x].size() - 1;
x = tata[x];
}
cout << ans << '\n';
return 0;
}
