#include <bits/stdc++.h>                                           
 
#define int long long
//#pragma GCC optimize("Ofast")
//#pragma comment(linker, "/stack:200000000")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4")
 
 
#define file(s) freopen(s".in","r",stdin); freopen(s".out","w",stdout);
#define forev(i, b, a) for(int i = (b); i >= (a); --i)
#define forn(i, a, b) for(int i = (a); i <= (b); ++i)
#define all(x) x.begin(), x.end()
#define sz(s) (int)s.size()
#define pb push_back
#define ppb pop_back
#define mp make_pair
#define s second
#define f first
 
 
using namespace std;
 
 
typedef pair < long long, long long > pll;    
typedef pair < int, int > pii;
typedef unsigned long long ull;         
typedef vector < pii > vpii;
typedef vector < int > vi;
typedef long double ldb;  
typedef long long ll;  
typedef double db;                             
 
 
const int dx[] = {1,	 -1, 0, 0}, dy[] = {0, 0, 1, -1}, block = 555;
const pii base = mp(1171, 3307), Mod = mp(1e9 + 7, 1e9 + 9);
const int inf = 1e9, maxn = 4e5 + 148, mod = 1e9 + 7, N = 1e4 + 211;
const db eps = 1e-12, pi = 3.14159265359;
const ll INF = 1e18;

int n, v, u, a, b, d1[N], d2[N];
vi g[N];	
queue < int > q;

main () {				
	cin >> n >> u >> v;
	forn (i, 1, n - 1) {
		cin >> a >> b;
		g[a].pb(b);
		g[b].pb(a);
	}	        	

	memset(d1, -1, sizeof(d1));
	q.push(v);
	d1[v] = 0;
	while (sz(q)) {
		int vv = q.front();
		q.pop();
		for (auto to : g[vv])
			if (d1[to] == -1) {
				d1[to] = d1[vv] + 1;
				q.push(to);
			}
		
	}

	memset(d2, -1, sizeof(d2));
	q.push(u);
	d2[u] = 0;
	while (sz(q)) {
		int vv = q.front();
		q.pop();
		for (auto to : g[vv])
			if (d2[to] == -1) {
				d2[to] = d2[vv] + 1;
				q.push(to);
			}
	}

	int ans = 0;
	forn (i, 1, n)
		if (d1[i] >= d2[i]) {
			ans = max(ans, d1[i] - 1); 
		}


	cout << ans << endl;
}                  	