#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define pii pair<int,int>
#define f first
#define s second
#define chmin(a, b) a = min(a,b)
#define chmax(a, b) a = max(a,b)
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define F0R(i, a) for (int i = 0; i < (a); i++)
#define all(x) x.begin(),x.end()
#define v vector
const int MAX_N = 3e5;
v<int> adj[MAX_N];
v<int> path;
int a, b;
bool find_path(int cur, int par) {
for (int x : adj[cur]) {
if (x!=par) {
path.pb(x);
if (find_path(x,cur)) {
return true;
} else {
path.pop_back();
}
}
}
return (cur==b);
}
int dfs(int cur, int par, int p) {
v<int> dp;
for (int x : adj[cur]) {
if (x!=par && x!=p) {
dp.pb(dfs(x,cur,p));
}
}
int ret = 0;
sort(all(dp),greater<>());
for (int i=0;i<dp.size();i++) {
ret = max(ret,dp[i]+i+1);
}
return ret;
}
pair<int,int> check(int p) {
return make_pair(dfs(a,-1,path[p]),dfs(b,-1,path[p-1]));
}
int main() {
ios::sync_with_stdio(false); cin.tie(0);
int n; cin >> n >> a >> b;
a--; b--;
path.pb(a);
for (int i=0;i<n-1;i++) {
int c, d; cin >> c >> d;
adj[--c].pb(--d);
adj[d].pb(c);
}
find_path(a,-1);
int lo = 1, hi = path.size()-1, mid = (lo+hi)/2, ans = path.size()-1;
while (lo <= hi) {
pii res = check(mid);
if (res.f >= res.s) {
ans = mid;
hi = mid-1;
} else {lo = mid+1;}
mid = (lo+hi)/2;
}
pii cand1 = check(ans), cand2 = (ans==0 ? make_pair(INT_MAX,INT_MAX) : check(ans-1));
cout << min(max(cand1.f,cand1.s),max(cand2.f,cand2.s)) << '\n';
}
