#include <bits/stdc++.h>
#define whlie while
#define pb push_back
#define eb emplace
#define fi first
#define se second
#define rep(i,N) for(int i = 0; i < (N); i++)
#define repr(i,N) for(int i = (N) - 1; i >= 0; i--)
#define rep1(i,N) for(int i = 1; i <= (N) ; i++)
#define repr1(i,N) for(int i = (N) ; i > 0 ; i--)
#define each(x,v) for(auto& x : v)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
#define ini(...) int __VA_ARGS__; in(__VA_ARGS__)
#define inl(...) ll __VA_ARGS__; in(__VA_ARGS__)
#define ins(...) string __VA_ARGS__; in(__VA_ARGS__)
using namespace std; void solve();
using ll = long long; using vl = vector<ll>;
using vi = vector<int>; using vvi = vector< vector<int> >;
constexpr int inf = 1001001001;
constexpr ll infLL = (1LL << 61) - 1;
struct IoSetupNya {IoSetupNya() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); cerr << fixed << setprecision(10);} } iosetupnya;
template<typename T, typename U> inline bool amin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template<typename T, typename U> inline bool amax(T &x, U y) { return (x < y) ? (x = y, true) : false; }
template<typename T, typename U> ostream& operator <<(ostream& os, const pair<T, U> &p) { os << p.first << " " << p.second; return os; }
template<typename T, typename U> istream& operator >>(istream& is, pair<T, U> &p) { is >> p.first >> p.second; return is; }
template<typename T> ostream& operator <<(ostream& os, const vector<T> &v) { int s = (int)v.size(); rep(i,s) os << (i ? " " : "") << v[i]; return os; }
template<typename T> istream& operator >>(istream& is, vector<T> &v) { for(auto &x : v) is >> x; return is; }
void in(){} template <typename T,class... U> void in(T &t,U &...u){ cin >> t; in(u...);}
void out(){cout << "\n";} template <typename T,class... U> void out(const T &t,const U &...u){ cout << t; if(sizeof...(u)) cout << " "; out(u...);}
template<typename T>void die(T x){out(x); exit(0);}
#ifdef NyaanDebug
  #include "NyaanDebug.h"
  #define trc(...) do { cerr << #__VA_ARGS__ << " = "; dbg_out(__VA_ARGS__);} while(0)
  #define trca(v,N) do { cerr << #v << " = "; array_out(v , N);cout << endl;} while(0)
#else
  #define trc(...)
  #define trca(...)
  int main(){solve();}
#endif
using P = pair<int,int>; using vp = vector<P>;
constexpr int MOD = /**/ 1000000007; //*/ 998244353;
////////////////

namespace fast {
    // fast I/O by rsk0315 (update: 2019-08-08 07:02:47).
    // This version supports only integer inputs/outputs, single character
    // outputs, and string literal outputs.
    static size_t constexpr buf_size = 1 << 17;
    static size_t constexpr margin = 1;
    static char inbuf[buf_size + margin] = {};
    static char outbuf[buf_size + margin] = {};
    static __attribute__((aligned(8))) char minibuf[32];
    static size_t constexpr int_digits = 20;  // 18446744073709551615
    static uintmax_t constexpr digit_mask = 0x3030303030303030;
    static uintmax_t constexpr first_mask = 0x00FF00FF00FF00FF;
    static uintmax_t constexpr second_mask = 0x0000FFFF0000FFFF;
    static uintmax_t constexpr third_mask = 0x00000000FFFFFFFF;
    static uintmax_t constexpr tenpow[] = {
            1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000
    };
    template <typename Tp>
    using enable_if_integral = std::enable_if<std::is_integral<Tp>::value, Tp>;
 
    class scanner {
        char* pos = inbuf;
        char* endpos = inbuf + buf_size;
 
        void M_read_from_stdin() {
            endpos = inbuf + fread(pos, 1, buf_size, stdin);
        }
        void M_reread_from_stdin() {
            ptrdiff_t len = endpos - pos;
            if (!(inbuf + len <= pos)) return;
            memcpy(inbuf, pos, len);
            char* tmp = inbuf + len;
            endpos = tmp + fread(tmp, 1, buf_size-len, stdin);
            *endpos = 0;
            pos = inbuf;
        }
 
    public:
        scanner() { M_read_from_stdin(); }
 
        template <typename Integral,
                typename enable_if_integral<Integral>::type* = nullptr>
        void scan_parallel(Integral& x) {
            // See https://qiita.com/rsk0315_h4x/items/17a9cb12e0de5fd918f4
            if (__builtin_expect(endpos <= pos + int_digits, 0))
                M_reread_from_stdin();
            bool ends = false;
            typename std::make_unsigned<Integral>::type y = 0;
            bool neg = false;
            if (std::is_signed<Integral>::value && *pos == '-') {
                neg = true;
                ++pos;
            }
            do {
                memcpy(minibuf, pos, 8);
                long c = *(long*)minibuf;
                long d = (c & digit_mask) ^ digit_mask;
                int skip = 8;
                int shift = 8;
                if (d) {
                    int ctz = __builtin_ctzl(d);
                    if (ctz == 4) break;
                    c &= (1L << (ctz-5)) - 1;
                    int discarded = (68-ctz) / 8;
                    shift -= discarded;
                    c <<= discarded * 8;
                    skip -= discarded;
                    ends = true;
                }
                c |= digit_mask;
                c ^= digit_mask;
                c = ((c >> 8) + c*10) & first_mask;
                c = ((c >> 16) + c*100) & second_mask;
                c = ((c >> 32) + c*10000) & third_mask;
                y = y*tenpow[shift] + c;
                pos += skip;
            } while (!ends);
            x = (neg? -y: y);
            ++pos;
        }
 
        template <typename Integral,
                typename enable_if_integral<Integral>::type* = nullptr>
        void scan_serial(Integral& x) {
            if (__builtin_expect(endpos <= pos + int_digits, 0))
                M_reread_from_stdin();
            bool neg = false;
            if (std::is_signed<Integral>::value && *pos == '-') {
                neg = true;
                ++pos;
            }
            typename std::make_unsigned<Integral>::type y = *pos-'0';
            while (*++pos >= '0') y = 10*y + *pos-'0';
            x = (neg? -y: y);
            ++pos;
        }
 
        template <typename Integral,
                typename enable_if_integral<Integral>::type* = nullptr>
        // Use scan_parallel(x) only when x may be too large (about 10^12).
        // Otherwise, even when x <= 10^9, scan_serial(x) may be faster.
        void scan(Integral& x) { scan_serial(x); }
    };
 
    class printer {
        char* pos = outbuf;
 
        void M_flush_stdout() {
            fwrite(outbuf, 1, pos-outbuf, stdout);
            pos = outbuf;
        }
 
    public:
        printer() = default;
        ~printer() { M_flush_stdout(); }
 
        void print(char c) {
            if (__builtin_expect(pos + 1 >= outbuf + buf_size, 0)) M_flush_stdout();
            *pos++ = c;
        }
 
        template <size_t N>
        void print(char const(&s)[N]) {
            if (__builtin_expect(pos + N >= outbuf + buf_size, 0)) M_flush_stdout();
            memcpy(pos, s, N-1);
            pos += N-1;
        }
 
        template <typename Integral,
                typename enable_if_integral<Integral>::type* = nullptr>
        void print(Integral x) {
            if (__builtin_expect(pos + int_digits >= outbuf + buf_size, 0))
                M_flush_stdout();
            if (x == 0) {
                *pos++ = '0';
                return;
            }
            if (x < 0) {
                *pos++ = '-';
                if (__builtin_expect(x == std::numeric_limits<Integral>::min(), 0)) {
                    switch (sizeof x) {
                        case 2: print("32768"); return;
                        case 4: print("2147483648"); return;
                        case 8: print("9223372036854775808"); return;
                    }
                }
                x = -x;
            }
            char* mp = minibuf + sizeof minibuf;
            *--mp = x % 10 + '0';
            size_t len = 1;
            typename std::make_unsigned<Integral>::type y = x / 10;
            while (y > 0) {
                *--mp = y % 10 + '0';
                y /= 10;
                ++len;
            }
            memcpy(pos, mp, len);
            pos += len;
        }
 
        template <typename Tp>
        void println(Tp x) { print(x), print('\n'); }
    };
} // fast::
 
fast::scanner fastin;
fast::printer fastout;

int N , x , y , T , A, ans = 0;
vi g[100010];
int td[100010] , ad[100010] , ok[100010];
stack<P> s;
 
void solve(){
  fastin.scan(N);
  fastin.scan(T);
  fastin.scan(A);
  T--; A--;
  rep(i , N - 1){
    fastin.scan(x); fastin.scan(y);
    x--; y--; g[x].pb(y); g[y].pb(x);
  }
  
  s.emplace(A , -1);
  while(!s.empty()){
    P cur = s.top(); s.pop();
    if(cur.se != -1) ad[cur.fi] = ad[cur.se] + 1;
    each(dst , g[cur.fi]){
      if(dst != cur.se) s.emplace(dst , cur.fi);
    }
  }
  s.emplace(P(T , -1));
  while(!s.empty()){
    P cur = s.top(); s.pop();
    if(cur.se != -1) td[cur.fi] = td[cur.se] + 1;
    ok[cur.fi] = td[cur.fi] < ad[cur.fi];
    if(cur.se != -1) ok[cur.fi] &= ok[cur.se];
    if(ok[cur.fi]) ans = max(ans , ad[cur.fi] - 1);
    each(dst , g[cur.fi]){
      if(dst != cur.se) s.emplace(dst , cur.fi);
    }
  }
  
  fastout.println(ans); 
}
 

Measured memory_bytes: 7168