#ifdef mmlang_source_32_lines

N = inputInt()
T[:N] = 0
U = queue!(int)()
V = queue!(int)()
U.emplace(inputInt() - 1)
V.emplace(inputInt() - 1)
E[:N] = vector!(int)()
if True or (: N - 1):
    a, b = inputInt() - 1, inputInt() - 1
    E[a].emplace_back(b)
    E[b].emplace_back(a)

turn = 0
cnt = 0
while True:
    assert not (U.empty() and V.empty())
    for u in move(U):
        T[u] |= 1
        if !(T[u] & 2):
            ++cnt
        if !(T[e@E[u][:]] & 1):
            U.emplace(e)
    for v in move(V):
        T[v] |= 2
        if T[v] & 1:
            --cnt
        if !(T[e@E[v][:]] & 2):
            V.emplace(e)
    if cnt == 0:
        break
    ++turn
print(turn - 1)

#endif

//#define NDEBUG

#include <cassert>
#include <vector>
#include <deque>
#include <string>
#include <map>
#include <queue>
#include <stack>
#include <algorithm>
#include <cstdlib>
#include <cstdio>
#include <cstring>

using namespace std;

typedef long long int64;typedef __int128 int128;const char*inputCLineOrWord(int mode){static char buf[2097152];static int bufLen=sizeof(buf);static int bufPos=sizeof(buf);static bool canReadFlag=true;static bool crFlag=false;static bool enterFlag=false;if(canReadFlag&&(enterFlag?bufLen<=bufPos:(int)sizeof(buf)<=bufPos+bufPos)){if(0<bufLen-bufPos){memmove(buf,buf+bufPos,bufLen-bufPos);bufLen-=bufPos;}else{bufLen=0;}char*result=fgets(buf+bufLen,sizeof(buf)-bufLen,stdin);canReadFlag=(result!=NULL);if(result!=NULL){int n=strlen(result);enterFlag=(n!=(int)sizeof(buf)-1-bufLen||(1<=bufLen+n&&buf[bufLen+n-1]=='\n'));bufLen+=n;}bufPos=0;}if(mode==0){int pos=bufPos;while(true){char c=buf[pos];if(c==32){buf[pos++]='\0';break;}else if(c==10){if(crFlag){crFlag=false;if(bufPos==pos){pos=++bufPos;continue;}}buf[pos++]='\0';break;}else if(c==13){crFlag=true;buf[pos++]='\0';break;}else if(c==0){break;}++pos;}const char*ret=buf+bufPos;bufPos=pos;while(true){char c=buf[bufPos];if(c==32||c==10||c==13){++bufPos;}else{break;}}return ret;}else if(mode==1){int pos=bufPos;while(true){char c=buf[pos];if(c==10){if(crFlag){crFlag=false;if(bufPos==pos){pos=++bufPos;continue;}}buf[pos++]='\0';break;}else if(c==13){crFlag=true;buf[pos++]='\0';break;}else if(c==0){break;}++pos;}const char*ret=buf+bufPos;bufPos=pos;if(crFlag){while(true){char c=buf[bufPos];if(c==13){++bufPos;crFlag=false;break;}else{break;}}}return ret;}else if(mode==2){return bufLen<=bufPos?NULL:buf+bufPos;}assert(false);return NULL;}const char*inputCWord(){return inputCLineOrWord(0);}int inputInt(){return atoi(inputCWord());}void print_unit(bool val){printf("%s",val?"true":"false");}void print_unit(char val){printf("%c",val);}void print_unit(int val){printf("%d",val);}void print_unit(unsigned int val){printf("%u",val);}void print_unit(size_t val){printf("%zd",val);}void print_unit(const void*val){printf("%p",val);}void print_unit(long long val){printf("%lld",val);}void print_unit(__int128 val){char buf[128];int idx=128;buf[--idx]='\0';bool sign=false;if(val<0){sign=true;val=-val;}if(val==0){buf[--idx]='0';}else while(val){buf[--idx]='0'+(val % 10);val /=10;}if(sign){buf[--idx]='-';}printf("%s",buf+idx);}void print_unit(double val){printf("%g",val);}void print_unit(const char*val){printf("%s",val);}void print_unit(const std::string&val){printf("%s",val.c_str());}template<class T>void print_unit(const std::vector<T>&val){printf("vec(%d) {",(int)val.size());for(int i=0;i<(int)val.size();++i){fputc(' ',stdout);print_unit(val[i]);}printf(" }");}
#define _print0()
#define _print1(e)print_unit(e)
#define _print2(e1,e2)_print1(e1),fputc(' ',stdout),_print1(e2)
#define _print3(e1,e2,e3)_print2(e1,e2),fputc(' ',stdout),_print1(e3)
#define _print4(e1,e2,e3,e4)_print2(e1,e2),fputc(' ',stdout),_print2(e3,e4)
#define _print5(e1,e2,e3,e4,e5)_print3(e1,e2,e3),fputc(' ',stdout),_print2(e4,e5)
#define _print6(e1,e2,e3,e4,e5,e6)_print3(e1,e2,e3),fputc(' ',stdout),_print3(e4,e5,e6)
#define _print7(e1,e2,e3,e4,e5,e6,e7)_print4(e1,e2,e3,e4),fputc(' ',stdout),_print3(e5,e6,e7)
#define _print8(e1,e2,e3,e4,e5,e6,e7,e8)_print4(e1,e2,e3,e4),fputc(' ',stdout),_print4(e5,e6,e7,e8)
#define _print9(e1,e2,e3,e4,e5,e6,e7,e8,e9)_print5(e1,e2,e3,e4,e5),fputc(' ',stdout),_print4(e6,e7,e8,e9)
#define _print10(e1,e2,e3,e4,e5,e6,e7,e8,e9,e10)_print5(e1,e2,e3,e4,e5),fputc(' ',stdout),_print5(e6,e7,e8,e9,e10)
#define _GET_PRINT_MACRO_NAME(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,NAME,...)NAME
#define print(...)_GET_PRINT_MACRO_NAME(__VA_ARGS__,_print10,_print9,_print8,_print7,_print6,_print5,_print4,_print3,_print2,_print1,_print0)(__VA_ARGS__),fputc('\n',stdout)
#define print0(...)_GET_PRINT_MACRO_NAME(__VA_ARGS__,_print10,_print9,_print8,_print7,_print6,_print5,_print4,_print3,_print2,_print1,_print0)(__VA_ARGS__)

// generated code (by mmlang) :

int N;
vector<int> T;
queue<int> U;
queue<int> V;
vector<vector<int> > E;
int a;
int b;
int turn;
int cnt;

int main() {
    N = inputInt();
    {
        T.resize(max((int)T.size(), (int)N));
        for(int $1=0, $e=(0<=N ? N : (int)T.size()+N); $1<$e; ++$1) T[$1] = 0;
    }
    U = queue<int>();
    V = queue<int>();
    U.emplace(inputInt() - 1);
    V.emplace(inputInt() - 1);
    {
        E.resize(max((int)E.size(), (int)N));
        for(int $1=0, $e=(0<=N ? N : (int)E.size()+N); $1<$e; ++$1) E[$1] = vector<int>();
    }
    for(int $e=N - 1, $1=0; $1<$e; ++$1) if(true || ($1)) {
        {
            const auto & $c1 = inputInt() - 1;
            const auto & $c2 = inputInt() - 1;
            tie(a, b) = tuple<int, int>($c1, $c2);
        }
        E[a].emplace_back(b);
        E[b].emplace_back(a);
    }
    turn = 0;
    cnt = 0;
    while(true) {
        assert(!(U.empty() && V.empty()));
        {
            auto $c1 = move(U);
            while(!($c1).empty()) { int u = ($c1).front(); ($c1).pop();
                T[u] |= 1;
                if(!(T[u] & 2)) {
                    ++cnt;
                }
                for(int $1=0, $e=(int)E[u].size(); $1<$e; ++$1) {
                    const auto & e = E[u][$1];
                    if(!(T[e] & 1)) {
                        U.emplace(e);
                    }
                }
            }
        }
        {
            auto $c1 = move(V);
            while(!($c1).empty()) { int v = ($c1).front(); ($c1).pop();
                T[v] |= 2;
                if(T[v] & 1) {
                    --cnt;
                }
                for(int $1=0, $e=(int)E[v].size(); $1<$e; ++$1) {
                    const auto & e = E[v][$1];
                    if(!(T[e] & 2)) {
                        V.emplace(e);
                    }
                }
            }
        }
        if(cnt == 0) {
            goto $L1;
        }
        ++turn;
    }
    $L1:;
    print(turn - 1);
    return 0;
}


Measured memory_bytes: 8792