import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

public class Main implements Runnable {
    public static void main(String[] args) {
        new Thread(null,new Main(), "" ,Runtime.getRuntime().maxMemory()).start();
    }

    // メイン処理
    public void run() {
        FastScanner sc = new FastScanner();
        int n = sc.nextInt();
        int u = sc.nextInt() - 1;
        int v = sc.nextInt() - 1;
        Map<Integer, Set<Integer>> routeMap = new HashMap<>(n);
        for(int i = 0;i < n - 1;i++) {
            int a = sc.nextInt() - 1;
            int b = sc.nextInt() - 1;
            Set<Integer> fromA = routeMap.get(Integer.valueOf(a));
            if(fromA == null) fromA = new HashSet<>();
            fromA.add(Integer.valueOf(b));
            routeMap.put(Integer.valueOf(a), fromA);
            Set<Integer> fromB = routeMap.get(Integer.valueOf(b));
            if(fromB == null) fromB = new HashSet<>();
            fromB.add(Integer.valueOf(a));
            routeMap.put(Integer.valueOf(b), fromB);
        }

        Deque<Integer> que = new ArrayDeque<>();
        Set<Integer> footprint = new HashSet<>(n);
        int[] distanceFromUto = new int[n];
        que.offer(Integer.valueOf(u));
        footprint.add(Integer.valueOf(u));
        while(!que.isEmpty()) {
            Integer here = que.poll();
            for(Integer next : routeMap.get(here)) {
                if(footprint.contains(next)) continue;
                distanceFromUto[next.intValue()] = distanceFromUto[here.intValue()] + 1;
                que.offer(next);
                footprint.add(next);
            }
        }

        que.clear();
        footprint.clear();
        int[] distanceFromVto = new int[n];
        que.offer(Integer.valueOf(v));
        footprint.add(Integer.valueOf(v));
        while(!que.isEmpty()) {
            Integer here = que.poll();
            for(Integer next : routeMap.get(here)) {
                if(footprint.contains(next)) continue;
                distanceFromVto[next.intValue()] = distanceFromVto[here.intValue()] + 1;
                que.offer(next);
                footprint.add(next);
            }
        }
        
        // アオキから一番遠い点かつタカハシがアオキよりも早く到着できる点を導出
        int farest = -1;
        int farIdx = -1;
        for(int i = 0;i < n;i++) {
            if(farest < distanceFromVto[i] && distanceFromUto[i] < distanceFromVto[i]) {
                farest = distanceFromVto[i];
                farIdx = i;
            }
        }
        int target = -1;
        for(Integer t : routeMap.get(Integer.valueOf(farIdx))) target = t.intValue();
        
        PrintWriter out =  new PrintWriter(System.out);
        out.println(distanceFromVto[target]);
        out.flush();
    }

    // 以下、ユーティリティ
    Set<long[]> primeFactorize(long n){
        // たとえnが10^18でも約数の個数は400を超えないため素因数はもっと少なくなる
        Set<long[]> ret = new HashSet<>(500);
        for(long p = 2;p * p <= n;p++) {
            if(n % p != 0) continue;
            long num = 0;
            while(n % p == 0) {
                ++num;
                n /= p;
            }
            long[] arr = {p, num};
            ret.add(arr);
        }
        if(n != 1) {
            long[] arr = {n, 1};
            ret.add(arr);
        }

        return ret;
    }

    // nの約数を求める
    Set<Long> searchDivisor(long n){
        Set<Long> ret = new HashSet<>(200);
        for(long i = 1;i * i <= n;i++) {
            if(n % i == 0) {
                ret.add(Long.valueOf(i));
                if(i * i != n) ret.add(Long.valueOf(n / i));
            }
        }

        return ret;
    }

    // 素数判定
    public static boolean isPrime(long num) {
        if (num < 2) return false;
        else if (num == 2) return true;
        else if (num % 2 == 0) return false;

        double sqrtNum = Math.sqrt(num);
        for (int i = 3; i <= sqrtNum; i += 2) {
            if (num % i == 0) return false;
        }

        return true;
    }

    long nCm(long n, long m) {
        if(n < m) return 0l;
        long c = 1l;
        m = (n - m < m ? n - m : m);
        for(long ns = n - m + 1, ms = 1;ms <= m;ns ++, ms++) {
            c *= ns;
            c /= ms;
        }
        return c;
    }

    long modnCm(long n, long m, long mod) {
        if(n < m) return 0l;
        long upper = 1l;
        long downer = 1l;
        m = (n - m < m ? n - m : m);
        for(long ns = n - m + 1, ms = 1;ms <= m;ns ++, ms++) {
            upper = upper * ns % mod;
            downer = downer * ms % mod;
        }
        return upper * modInv(downer, mod) % mod;
    }

    // mod m での a の逆元 を計算する
    long modInv(long a, long m) {
        long b = m, u = 1, v = 0;
        while(b > 0) {
            long t = a / b;
            a -= t * b;
            long tmp = a;
            a = b;
            b = tmp;
            u -= t * v;
            tmp = u;
            u = v;
            v = tmp;
        }
        u %= m;
        if (u < 0) u += m;
        return u;
    }

    long modPow(long a, long n, long mod) {
        long res = 1l;
        while(n > 0l) {
            if(n % 2l == 1l) res = res * a % mod;
            a = a * a % mod;
            n /= 2l;
        }
        return res;
    }

}

class UnionFind {
    protected int[] parent;

    UnionFind(int n) {
        parent = new int[n];
        for(int i = 0;i < n;i++) parent[i] = i;
    }

    public int rootOf(int x) {
        if(parent[x] == x) return x;
        else return parent[x] = rootOf(parent[x]);
    }

    public boolean same(int x, int y) {
        return rootOf(x) == rootOf(y);
    }

    public void unite(int x, int y) {
        x = rootOf(x);
        y = rootOf(y);
        if(x == y) return;
        parent[x] = y;
        return;
    }
}

class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
    private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
    public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double nextDouble() { return Double.parseDouble(next());}
}

Measured memory_bytes: 75640