#include <bits/stdc++.h>
using namespace std;
const int MAXN = 3e5 + 7;
int a,b,n;
vector<int> susjedi[MAXN], path;
int cost[MAXN];
pair<int, int> obris;
vector<int> v[MAXN];
bool put (int x, int p, int y) {
if (x == y) {
path.push_back (x);
return 1;
}
for (auto nx: susjedi[x]) {
if (nx == p) continue;
if (put (nx, x, y)) {
path.push_back (x);
return 1;
}
}
return 0;
}
void dfs (int x, int p) {
cost[x] = 0;
for (auto nx: susjedi[x]) {
if (nx == p) continue;
if ((obris.first == x && obris.second == nx) || (obris.first == nx && obris.second == x)) continue;
dfs (nx, x);
v[x].push_back (cost[nx]);
}
sort (v[x].begin(), v[x].end ());
int cnt = 1;
for (int i = v[x].size () - 1; i >= 0; i--) {
cost[x] = max (cost[x], v[x][i] + cnt);
cnt++;
}
}
void clean () {
for (int i = 0; i < n; i++) v[i].clear ();
}
int solve (int k) {
obris = {path[k], path[k + 1]};
clean ();
dfs (a, -1);
dfs (b, -1);
return max (cost[a], cost[b]);
}
int ts () {
int lo = 0, hi = path.size() - 2;
while (lo < hi) {
int mid = (lo + hi) / 2;
if (solve (mid) < solve (mid + 1)) hi = mid;
else lo = mid + 1;
}
return solve (lo);
}
int main () {
ios_base::sync_with_stdio(false);
cin.tie (0);
cin >> n >> a >> b;
a--;b--;
for (int i = 0; i < n - 1; i++) {
int x, y;
cin >> x >> y;
x--;y--;
susjedi[x].push_back (y);
susjedi[y].push_back (x);
}
put (a, -1, b);
cout << ts ();
return 0;
}
