import java.util.*; import java.io.*; import java.math.*;
public class Main{
	static Node[] list;
	static void solve(){//Here is the main function
		ArrayList<Integer> one = nextIntArray();
		int N = one.get(0);
		int u = one.get(1) - 1;
		int v = one.get(2) - 1;
		list = new Node[N];
		for(int i = 0; i < N; i++){
			list[i] = new Node();
		}
		for(int i = 0; i < N - 1; i++){
			ArrayList<Integer> tmp = nextIntArray();
			list[tmp.get(0) - 1].child.add(tmp.get(1) - 1);
			list[tmp.get(1) - 1].child.add(tmp.get(0) - 1);
		}
		bfs(u, true);
		bfs(v, false);
		int max = 0;
		for(int i = 0; i < N; i++){
			if(list[i].tcount < list[i].acount){
				max = Math.max(max, list[i].acount - 1);
			}
		}
		myout(max);
	}
	//Method addition frame start

static void bfs(int s, boolean isT){
	ArrayDeque<Integer> queue = new ArrayDeque<>();
	queue.add(s);
	if(isT){
		list[s].taccess = true;
	}else{
		list[s].aaccess = true;
	}
	while(queue.size() > 0){
		int now = queue.pollFirst();
		ArrayList<Integer> child = list[now].child;
		for(int i = 0; i < child.size(); i++){
			if(isT){
				if(!list[child.get(i)].taccess){
					list[child.get(i)].taccess = true;
					list[child.get(i)].tcount = list[now].tcount + 1;
					queue.add(child.get(i));
				}
			}else{
				if(!list[child.get(i)].aaccess){
					list[child.get(i)].aaccess = true;
					list[child.get(i)].acount = list[now].acount + 1;
					queue.add(child.get(i));
				}
			}
		}
	}
}

static class Node{
	ArrayList<Integer> child = new ArrayList<>();
	int tcount = 0;
	int acount = 0;
	boolean taccess = false;
	boolean aaccess = false;
	Node(){}
}

	//Method addition frame end

	//Don't have to see. start------------------------------------------
	static class InputIterator{
		ArrayList<String> inputLine = new ArrayList<String>(1024);
		int index = 0; int max; String read;
		InputIterator(){
			BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
			try{
				while((read = br.readLine()) != null){
					inputLine.add(read);
				}
			}catch(IOException e){}
			max = inputLine.size();
		}
		boolean hasNext(){return (index < max);}
		String next(){
			if(hasNext()){
				return inputLine.get(index++);
			}else{
				throw new IndexOutOfBoundsException("There is no more input");
			}
		}
	}
	static HashMap<Integer, String> CONVSTR = new HashMap<Integer, String>();
	static InputIterator ii = new InputIterator();//This class cannot be used in reactive problem.
	static PrintWriter out = new PrintWriter(System.out);
	static void flush(){out.flush();}
	static void myout(Object t){out.println(t);}
	static void myerr(Object t){System.err.print("debug:");System.err.println(t);}
	static String next(){return ii.next();}
	static boolean hasNext(){return ii.hasNext();}
	static int nextInt(){return Integer.parseInt(next());}
	static long nextLong(){return Long.parseLong(next());}
	static double nextDouble(){return Double.parseDouble(next());}
	static ArrayList<String> nextStrArray(){return myconv(next(), 8);}
	static ArrayList<String> nextCharArray(){return myconv(next(), 0);}
	static ArrayList<Integer> nextIntArray(){
		ArrayList<String> input = nextStrArray(); ArrayList<Integer> ret = new ArrayList<Integer>(input.size());
		for(int i = 0; i < input.size(); i++){
			ret.add(Integer.parseInt(input.get(i)));
		}
		return ret;
	}
	static ArrayList<Long> nextLongArray(){
		ArrayList<String> input = nextStrArray(); ArrayList<Long> ret = new ArrayList<Long>(input.size());
		for(int i = 0; i < input.size(); i++){
			ret.add(Long.parseLong(input.get(i)));
		}
		return ret;
	}
	static String myconv(Object list, int no){//only join
		String joinString = CONVSTR.get(no);
		if(list instanceof String[]){
			return String.join(joinString, (String[])list);
		}else if(list instanceof ArrayList){
			return String.join(joinString, (ArrayList)list);
		}else{
			throw new ClassCastException("Don't join");
		}
	}
	static ArrayList<String> myconv(String str, int no){//only split
		String splitString = CONVSTR.get(no);
		return new ArrayList<String>(Arrays.asList(str.split(splitString)));
	}
	public static void main(String[] args){
		CONVSTR.put(8, " "); CONVSTR.put(9, "\n"); CONVSTR.put(0, "");
		solve();flush();
	}
	//Don't have to see. end------------------------------------------
}


Measured memory_bytes: 79328