#include <bits/stdc++.h>
using namespace std;
#define ar array
#define ll long long
#define int long long
const int inf = 2e9;
void solve() {
int n, a, b;
cin >> n >> a >> b;
--a, --b;
vector<vector<int>> g(n);
for (int i = 1; i < n;i++) {
int u, v;
cin >> u >> v;
--u, --v;
g[u].push_back(v);
g[v].push_back(u);
}
vector<int> dpA(n, 0), parA(n, -1);
function<void(int, int)> dfsA = [&](int u, int p) {
vector<int> arr;
for (int v : g[u]) {
if (v == p)
continue;
dfsA(v, u);
parA[v] = u;
if (v != b)
arr.push_back(dpA[v]);
}
sort(arr.rbegin(), arr.rend());
for (int i = 0; i < arr.size(); i++)
dpA[u] = max(dpA[u], i + 1 + arr[i]);
};
dfsA(a, -1);
vector<int> dpB(n, 0), parB(n, -1);
function<void(int, int)> dfsB = [&](int u, int p) {
vector<int> arr;
for (int v : g[u]) {
if (v == p)
continue;
dfsB(v, u);
parB[v] = u;
if (v != a)
arr.push_back(dpB[v]);
}
sort(arr.rbegin(), arr.rend());
for (int i = 0; i < arr.size(); i++)
dpB[u] = max(dpB[u], i + 1 + arr[i]);
};
dfsB(b, -1);
vector<int> path;
int cur = parB[a];
while (cur != b)
path.push_back(cur), cur = parB[cur];
reverse(path.begin(), path.end());
auto f = [&](int x) {
if (x >= path.size() | x < 0)
return inf;
auto tmpA = dpA;
auto tmpB = dpB;
x = path[x];
int z = parA[x];
while (z != -1) {
vector<int> arr;
for (int v : g[z]) {
if (v == parA[z] || v == x)
continue;
arr.push_back(dpA[v]);
}
dpA[z] = 0;
sort(arr.rbegin(), arr.rend());
for (int i = 1; i <= arr.size(); i++)
dpA[z] = max(dpA[z], i + arr[i - 1]);
z = parA[z];
}
z = x;
while (z != -1) {
vector<int> arr;
for (int v : g[z]) {
if (v == parB[z] || v == parA[x])
continue;
arr.push_back(dpB[v]);
}
dpB[z] = 0;
sort(arr.rbegin(), arr.rend());
for (int i = 1; i <= arr.size(); i++)
dpB[z] = max(dpB[z], i + arr[i - 1]);
z = parB[z];
}
int res = max(dpA[a], dpB[b]);
dpA = tmpA;
dpB = tmpB;
return res;
};
int ans = max(dpA[a], dpB[b]);
// for (int i = 0; i < path.size();i++)
// ans = min(ans, f(i));
int l = 0, r = path.size() - 1;
while (l < r) {
int m = (l + r) / 2;
int x1 = f(m);
int x2 = f(m + 1);
ans = min({ans, x1, x2});
if (x1 >= x2)
l = m + 1;
else
r = m;
}
ans = min({ans, f(l), f(r)});
cout << ans << "\n";
}
int32_t main() {
#ifdef Behruz
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios :: sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
solve();
return 0;
}
