#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 1e6;
bool onPathTM[MAX_N + 1];
int parent[MAX_N + 1];
int dpArb[MAX_N + 1];
vector<int> adj[MAX_N + 1];
void dfs(int u, int p) {
parent[u] = p;
for (int v: adj[u]) {
if (v == p)
continue;
dfs(v, u);
}
}
void calcDp(int u, int p) {
int max1 = 0, max2 = 0;
int sons = 0;
for (int v: adj[u]) {
if (v == p)
continue;
if (!onPathTM[v]) {
calcDp(v, u);
if (dpArb[v] > max1) {
max2 = max1;
max1 = dpArb[v];
} else if (dpArb[v] > max2)
max2 = dpArb[v];
sons++;
}
}
dpArb[u] = sons + max2;
// printf("%d %d\n", u, dp[u]);
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n, t, m;
cin >> n >> t >> m;
for (int e = 0; e < n - 1; e++) {
int u, v;
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
dfs(t, 0);
int w = m;
vector<int> path;
while (w != t) {
onPathTM[w] = true;
path.push_back(w);
w = parent[w];
}
onPathTM[w] = true;
int totalSons = 0;
for (int u: path) {
for (int v: adj[u]) {
if (onPathTM[v])
continue;
totalSons++;
}
}
calcDp(t, 0);
int l = -1, r = n;
while (r - l > 1) {
int mid = (l + r) / 2;
int b = 0, remSons = totalSons;
int a = 0;
bool ok = true;
for (int u: path) {
b++;
int crtSons = 0, aa = 0;
for (int v: adj[u]) {
if (onPathTM[v])
continue;
// printf("%d %d %d\n", dpArb[v], remSons, a);
if (dpArb[v] + remSons + a > mid) {
b--;
aa++;
if (b < 0)
ok = false;
}
crtSons++;
}
remSons -= crtSons;
a += aa;
}
if (a > mid)
ok = false;
// printf("%d %d\n", mid, ok);
if (ok)
r = mid;
else
l = mid;
}
cout << r << "\n";
return 0;
}
