/// Bismillahir Rahmanir Rahim
//Author: Tanvir Hussain
//ICE,NSTU
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
const long long MOD = 1000000007;
#define SET(x) memset(x, 0, sizeof(x))
#define SET2d(x,m,n) memset(x, 0, sizeof(x[0][0]) * m * n)
#define SETBOOL(x) memset(x,false,sizeof(x))
#define CLR(x) memset(x, -1, sizeof(x))
#define CLR2d(x,m,n) memset(x, -1, sizeof(x[0][0]) * m * n)
#define mp make_pair
#define PII pair<int, int>
#define pf printf
#define sf scanf
#define ALL(x) x.begin(),x.end()
#define pb push_back
#define IOS ios::sync_with_stdio(false); cin.tie(0);
#define np std::string::npos
#define highest(x) numeric_limits<x>::max()
#define lowest(x) numeric_limits<x>::min()
#define Inf INFINITY
#define minv(v) *min_element(v.begin(),v.end())
#define maxv(v) *max_element(v.begin(),v.end())
#define cases(cs,t) for(int cs=1;cs<=t;cs++)
#define PI acos(-1)
#define no1 __builtin_popcount
#define BOUNDARY(i, j) ((i >= 0 && i < row) && (j >= 0 && j < column))
#define uniq(vec) vec.resize(distance(vec.begin(),unique(vec.begin(),vec.end())))
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define sz(a) int(a.size())
#define ff first
#define ss second
#define endl "\n"
#define forch(it,s) for(auto it:s)
#define each(it,s) for(auto it = s.begin(); it != s.end(); ++it)
#define rep(i,a) for(int i=0; i<a;i++)
#define rep1(i,a,b) for(int i=(a);i<=(b);++i)
#define irep(i,b,a) for(int i=(b);i>=(a);--i)
#define bits(n) __builtin_popcount(n)
#define maxpq priority_queue<int>
#define minpq priority_queue<int, vector<int>, greater<int> >


int gcd(int a, int b) { if (a == 0) return b; return gcd(b % a, a);}
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<pair<int,int>> vpii;
typedef tree<pair<int, int>,null_type,less<pair<int, int>>,rb_tree_tag,tree_order_statistics_node_update> ordered_multiset;
int dx8[] = {0, 0, 1, 1, 1, -1, -1, -1};
int dy8[] = {1,-1, 1, -1, 0, 0, -1, 1};
int dx4[] = {0, 0, 1, -1};
int dy4[] = {1, -1, 0, 0};
const int maxx=100005;

//this fuction sorts vector pair according to first element in descending order.
bool sortinrev(const pair<int,int> &a,const pair<int,int> &b)
{
    return a.first>b.first;
}

template<typename T>inline T Bigmod(T base, T power, T MOD){
    T ret=1;
    while(power)
    {
        if(power & 1)ret=(ret*base)%MOD;
        base=(base*base)%MOD;
        power>>=1;
    }
    return ret;
}
double sq(double x) {return x*x;}
ll po(ll b,ll p){
    ll res=1;
    while(p){
        res*=b;
        p--;
    }
    return res;
}
ll lg2(ll x){
    ll res=0;
    while(x>1){
        res++;
        x/=2ll;
    }
    return res;
}
void solve(){
    int n,u,v;
    cin>>n>>u>>v;
    u--,v--;
    vi edg[n];
    rep(i,n-1){
        int u,v;
        cin>>u>>v;
        u--,v--;
        edg[u].pb(v);
        edg[v].pb(u);
    }
    vi taka(n,-1),akoi(n,-1);
    queue<int>q;
    taka[u]=0;
    q.push(u);
    while(!q.empty()){
        int u=q.front();
        q.pop();
        forch(it,edg[u]){
            if(taka[it]==-1){
                taka[it]=taka[u]+1;
                q.push(it);
            }
        }
    }
    akoi[v]=0;
    q.push(v);
    while(!q.empty()){
        int u=q.front();
        q.pop();
        forch(it,edg[u]){
            if(akoi[it]==-1){
                akoi[it]=akoi[u]+1;
                q.push(it);
            }
        }
    }
    int ans=0;
    rep(i,n){
        if(taka[i]<akoi[i]){
            ans=max(ans,akoi[i]);
        }
    }
    cout<<(ans-1)<<endl;
}

signed main()

{
    IOS;
    /*#ifndef ONLINE_JUDGE
        freopen ("data.in","r",stdin);
        freopen ("data.out","w",stdout);
    #endif*/
    int t;
    t=1;
    while(t--){
        solve();
    }

     return 0;

}
///Alhamdulillah



#include<bits/stdc++.h>
using namespace std;

vector<int>adj[100003],leaf;
int par[100003],depth[100005],sparse[100005][20],root=0,lz=0;

int lca(int p,int q)
{

    int a,b,c,d,lp,i,j;

    if(depth[p]<depth[q])
    {
        c=p; p=q; q=c;
    }

    for(j=19;j>=0;j--)
    {
         if((depth[p]-(1 << j))>=depth[q])
         {
             p=sparse[p][j];  if(depth[p]==depth[q])break;
         }
    }

    if(p==q){ return p; }
    else
    {
         for(j=19;j>=0;j--)
         {
             if(sparse[p][j]!=0 && sparse[p][j]!=sparse[q][j])
             {

                 p=sparse[p][j]; q=sparse[q][j];

                 if(par[p]==par[q])break;
             }

         }

         return par[p];
    }



}

void dfs(int src)
{
    int sz=adj[src].size();

    if(sz==1 && src!=root){   leaf.push_back(src);  lz++;   }

    for(int lp=0;lp<sz;lp++)
    {
        int u=adj[src][lp];

        if(u!=root && par[u]==0)
        {
            par[u]=src; sparse[u][0]=src; depth[u]=depth[src]+1;
            dfs(u);

        }

    }

}

int main()
{

    int n,m,a,b,i,j,u,v,ans,z,x,y,fr,sc,tr,w;
    scanf("%d %d %d",&n,&u,&v);

    for(i=1;i<=n-1;i++)
    {
        scanf("%d %d",&a,&b);
        adj[a].push_back(b); if(root==0){  if(adj[a].size()==2)root=a;  }
        adj[b].push_back(a); if(root==0){  if(adj[b].size()==2)root=b;  }

    }

    if(n==2)printf("0\n");
    else
    {
       dfs(root);

       for(j=1;j<=19;j++)
       {
           for(i=1;i<=n;i++)
           {
               if(sparse[i][j-1]!=0)
               {
                   sparse[i][j]=sparse[sparse[i][j-1]][j-1];
               }
           }
       }

       m=lca(u,v);

       ans=(depth[v]-depth[m])+(depth[u]-depth[m]); ans=ans-1;

       for(i=0;i<lz;i++)
       {
           y=leaf[i]; m=lca(u,y); w=lca(v,y);

           fr=(depth[u]-depth[m])+(depth[y]-depth[m]);
           sc=(depth[v]-depth[w])+(depth[y]-depth[w]);

           if(fr<sc)
           {
               if(sc-1>ans)ans=sc-1;
           }

       }

       printf("%d\n",ans);


    }



    return 0;
}
