#include <bits/stdc++.h>
#define int long long
#define ll long long
#define pb push_back
#define fi first
#define se second
#define lf (id<<1)
#define rg ((id<<1)|1)
#define md ((l+r)>>1)
#define ld long double
using namespace std;
typedef pair<int,int> pii;
typedef pair<char,char> pcc;
typedef pair<pii,int> ipii;
typedef pair<pii,pii> ipiii;
const int MAXN = 3e5+10;
const int MAXA = 1e7+10;
const int INF = 1e9+10;
const int SQRT = 300;
const int LOG = 20;
const ld EPS = 1e-6;
const int MOD = 1e9+7;
int sum(int a, int b){ return (a+b)%MOD; }
void chsum(int &a, int b){ a = (a+b)%MOD; }
void chsub(int &a, int b){ a = (a+MOD-b)%MOD; }
int mul(int a, int b){ return (a*b)%MOD; }
void chmul(int &a, int b){ a = (a*b)%MOD; }
void chmn(int &a, int b){ a = min(a, b); }
void chmx(int &a, int b){ a = max(a, b); }
int expo(int a, int b){
if(b==0) return 1;
int te = expo(a, b/2); te = mul(te,te);
return (b%2 ? mul(te,a) : te);
}
int n, A, B;
vector <int> adj[MAXN];
vector <int> path;
bool done = 0;
void precom(int nw, int par){
path.pb(nw);
if(nw==B){
done = 1; return;
}
for(auto nx : adj[nw]){
if(nx==par) continue;
precom(nx, nw);
if(done) return;
}
path.pop_back();
}
int dp[MAXN];
bool tag[MAXN];
void dfs(int nw, int par){
vector <int> vec;
for(auto nx : adj[nw]){
if(nx==par || tag[nx]) continue;
dfs(nx, nw);
vec.pb(dp[nx]);
}
sort(vec.rbegin(), vec.rend());
for(int i=0; i<vec.size(); i++)
chmx(dp[nw], vec[i]+i+1);
}
int ans = INF;
void calc(int i){
for(int j=1; j<=n; j++) dp[j] = 0;
tag[path[i+1]] = 1;
dfs(A,-1);
tag[path[i+1]] = 0;
tag[path[i]] = 1;
dfs(B, -1);
tag[path[i]] = 0;
chmn(ans, max(dp[A], dp[B]));
}
signed main(){
// ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin>>n>>A>>B;
for(int i=1; i<=n-1; i++){
int x, y; cin>>x>>y;
adj[x].pb(y); adj[y].pb(x);
}
precom(A, -1);
int l=0, r=path.size()-1, mid=0, cnt=0;
while(l<=r){
mid = md;
calc(mid);
if(dp[A] <= dp[B]) l = mid+1, cnt = mid;
else r = mid-1;
}
if(cnt+1 < path.size()-1) calc(cnt+1);
// for(int i=1; i<=n; i++)
// cout << i << ' ' << dp[i] << " dp\n";
cout << ans << '\n';
}
