/* ID: thanhvq1
LANG: C++14
PROB: namenum
*/

//#pragma GCC optimize ("O2")
#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#include<unistd.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
//using namespace __gnu_pbds;
//template <typename T>
//using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

// defines
#define int long long
typedef long long ll;
#define double long double
#define vec vector
#define vi vector<int>
#define vvi vector<vector<int>>
#define ii pair<int, int>
#define vii vector<ii>
#define F first
#define S second
#define MP make_pair
#define PB push_back
#define ALL(x) begin(x),end(x)
#define SZ(x) ((int)(x).size())
#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define FORE(i, a, b) for(int i = (a); i <= (b); i++)
#define FORD(i, a, b) for(int i = (a); i >= (b); i--)
//#define UNI(x) (x).resize(unique((x).begin(), (x).end()) - (x).end());
#define SORT_UNIQUE(c) (sort(c.begin(),c.end()), c.resize(distance(c.begin(),unique(c.begin(),c.end()))))
#define GET_POS(c,x) (lower_bound(c.begin(),c.end(),(x))-c.begin())
#define MS0(X) memset((X), 0, sizeof((X)))
#define MS1(X) memset((X), -1, sizeof((X)))
#define RESET(X, y) memset(X, y, sizeof(X))
#define chkmin(a, b) a = min(a, b)
#define chkmax(a, b) a = max(a, b)
#define max3(a, b, c) max(a, max(b, c))
#define min3(a, b, c) min(a, min(b, c))

//Universal DEBUG: https://codeforces.com/blog/entry/68809
void __print(int x) {cout << x;}
void __print(long x) {cout << x;}
//void __print(long long x) {cout << x;} //invalid if #define int long long
void __print(unsigned x) {cout << x;}
void __print(unsigned long x) {cout << x;}
void __print(unsigned long long x) {cout << x;}
void __print(float x) {cout << x;}
void __print(double x) {cout << x;}
//void __print(long double x) {cout << x;}
void __print(char x) {cout << '\'' << x << '\'';}
void __print(const char *x) {cout << '\"' << x << '\"';}
void __print(const string &x) {cout << '\"' << x << '\"';}
void __print(bool x) {cout << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cout << '{'; __print(x.first); cout << ','; __print(x.second); cout << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cout << '{'; for (auto &i: x) cout << (f++ ? "," : ""), __print(i); cout << "}";}
void _print() {cout << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cout << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define debug(x...) cout << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
/* To debug, just type: debug(x, y, z...). It requires C++ 11 or above.
It can work with: Primitive data types: bool, int, long long, float, ...; pair, string;
Collection types: std::vector, std::map, std::set, ...; Expressions
*/

//permanent constants
const long long INF64 = 1e18; const int INF32 = 1e9; const int BIG = (int)(1e9 + 239);
const int MOD = 1000000007; // (int)1e9+7; //998244353; const ll MOD2 = (ll)MOD * (ll)MOD;
const double PI = 2.0*acos(0.0); //acos(-1.0L);
const double EPS = static_cast<double>(1e-10);
const int day[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
const int dx[4] = {0, 1, 0, -1}; const int dy[4] = {1, 0, -1, 0};

//random
mt19937 rnd(239); //(chrono::high_resolution_clock::now().time_since_epoch().count());

typedef vector<vector<int>> matrix;   //matrix Z(n, vector<LL>(n));

/*GRAPH:
	vector<vector<int>> dist; // ma tran dist giua cac dinh
    dist.assign(N, vector<int>(N, INF)); // Khoi tao ma tran dist bang INF
*/
// Sort w Lambda Function: sort(begin(v), end(v), [] (int a, int b) { return a > b; });

void FILEIO() {  freopen("in.txt", "r", stdin);  freopen("out.txt", "w", stdout); }
//void FILEIO() {  freopen("lineup.in", "r", stdin);  freopen("lineup.out", "w", stdout); }
//Tricks:
#define odd(x) ((x) & 1)
#define even(x) !((x) & 1)

int mostSignDigit(int n) { double k = log10(n); return pow(10, k-floor(k));}
int numOfDigits(int n) {return floor(log10(n)) + 1;}
bool isPowerOfTwo(int x) {return x && (!(x&(x-1)));} // first x is for the case x=0

bool allPositive(vector<int> &a) {return all_of(begin(a), end(a), [](int x){return x > 0;});} // are all of the elements positive?
bool anyPositive(vector<int> &a) {return any_of(begin(a), end(a), [](int x){return x > 0;});} // is there at least one positive element?
bool nonePositive(vector<int> &a) {return none_of(begin(a), end(a), [](int x){return x > 0;});} // are none of the elements positive?

//******************
//*********BFS:
//const int maxN = 5*1e5;
vvi adj;  // adjacency list representation
int n; // number of nodes
vector<int> d, p; //d[u] - shortest dist from s to u, p[u] - parent of u
vector<bool> used;
int mx;

void bfs(int s) { // s - source vertex
	queue<int> q; q.push(s); used[s] = true;
	p[s] = -1; //no parent for s
	while (!q.empty()) {
	    int v = q.front(); q.pop();
	    for(int u : adj[v]) {
	        if (!used[u]) {
	            q.push(u); used[u] = true;
	            d[u] = d[v] + 1; // update distance for u
	            p[u] = v;  // update parent for u
	            mx = max(mx, d[u]);

//	            debug(mx, u, d[u]);

        	}
    	}
	}
}
//*********RESTORE SHORTEST PATH FROM BFS:
vector<int> path;
void restorePath(int u) { // u - destination (from source s as above)
	if (!used[u]) cout << "No path!";
	else
    	for (int v = u; v != -1; v = p[v]) path.push_back(v);
    		reverse(path.begin(), path.end());
}

void solve() {
	int u, v; cin >> n >> u >> v; u--, v--;
	if(u==v) {cout << 0 << endl; return;}
	adj.resize(n);
	FOR(i, 0, n-1) {
		int a, b; cin >> a >> b; a--, b--;
		adj[a].PB(b); adj[b].PB(a);
	}
	d.resize(n), p.resize(n); //d[u] - shortest dist from s to u, p[u] - parent of u
	used.assign(n, false);
	bfs(v); // bfs from v, so v is root
	restorePath(u); // get vi path as shortest path from v to u

//	debug(path);

	int m = SZ(path); // so dinh trong path tu v den u
	int ans = 0;
	if(even(m)) { // neu so dinh la chan, tuc la so doan la le
		int k = (m-2)/2; ans += k; // v tien den phia u m/2-1 buoc la toi da
		int e = path[k];
		int s = path[k+1];

//		debug(k, e, s);

		used.assign(n, false); used[e] = true;
		mx = 0; d.assign(n, 0);
		bfs(s);
		ans += mx;
	}
	else {
//		if(m==3) {cout<<1<<endl; return;}
		int k = (m-3)/2; ans += k+1; // v tien den phia u m/2-1 buoc la toi da
		int e = path[k+1]; // player 1 khong the den dinh e nay
		int s = path[k+2];

//		debug(k, e, s);

		used.assign(n, false); used[e] = true;
		mx = 0; d.assign(n, 0);
		bfs(s);
		ans += mx;
	}
	cout << ans << endl;
}

signed main() {
  ios_base::sync_with_stdio(false); cin.tie();
//  FILEIO();
//  auto beginProgram = chrono::steady_clock::now();

//  int t; cin >> t;
//  FOR(i, 0, t)
  {
//	  cout << "Case #" << i+1 << ": " << solve() << endl;
	  solve();
  }

//  auto endProgram = chrono::steady_clock::now();
//  cout << chrono::duration_cast<chrono::milliseconds>(endProgram - beginProgram).count() << " ms" << endl;

  return 0;
}


Measured memory_bytes: 8064