#include <bits/stdc++.h>
using namespace std;
#define repr(i,a,b) for(int i=a;i<b;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define reprrev(i,a,b) for(int i=b-1;i>=a;i--) // [a, b)
#define reprev(i,n) reprrev(i,0,n)
typedef long long ll;
typedef unsigned long long ull;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

const ll mod = 1e9+7;


void chmod(ll &M){
    if(M >= mod) M %= mod;
    else if(M < 0){
        M += (abs(M)/mod + 1)*mod;
        M %= mod;
    }
}

ll modpow(ll x, ll n){
    if(n==0) return 1;
    ll res=modpow(x, n/2);

    if(n%2==0) return res*res%mod;
    else return res*res%mod*x%mod;
}

int getl(int i, int N) { return i==0? N-1:i-1; };
int getr(int i, int N) { return i==N-1? 0:i+1; };

// 線分 ab の偏角 返り値は[-π, π]
double argument(const pair<double, double> &a, const pair<double, double> &b){
    double ax=a.first, ay=a.second, bx=b.first, by=b.second;
    return atan2(by-ay, bx-ax);
}

/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */

/* 注意 

    1LL<<60 * N とかのオーバーフローに気を付けろ
    制約がヒントになる、ちゃんと全ての制約を見ろ

*/

int n, u, v;
vector<vector<int>> adj;
vector<int> distFromAoki(100005, -1), distFromTakahashi(100005, -1);

void input() {
    cin >> n >> u >> v;
    --u; --v;
    adj.resize(n, vector<int>());
    rep(i, n-1) {
        int a, b; cin >> a >> b;
        --a; --b;
        adj[a].push_back(b);
        adj[b].push_back(a);
    }
}

void solve() {
    // fromAoki
    queue<int> q;
    q.push(v);
    distFromAoki[v] = 0;
    while(!q.empty()) {
        int now = q.front(); q.pop();
        for(auto to: adj[now]) {
            if (distFromAoki[to]==-1) {
                distFromAoki[to] = distFromAoki[now]+1;
                q.push(to);
            }
        }
    }

    // fromTakahashi
    queue<int> r;
    r.push(u);
    distFromTakahashi[u] = 0;
    while(!r.empty()) {
        int now = r.front(); r.pop();
        for(auto to: adj[now]) {
            if (distFromTakahashi[to]==-1 && distFromTakahashi[now]+1<distFromAoki[to]) {
                distFromTakahashi[to] = distFromTakahashi[now]+1;
                r.push(to);
            }
        }
    }

    int target;
    int tmpDist = -1;
    for(int i=0; i<n; ++i) {
        if (distFromTakahashi[i]!=-1 && distFromAoki[i]>tmpDist) {
            tmpDist = distFromAoki[i];
            target = i;
        }
    }
    cout << distFromAoki[target]-1 << endl;

}

int main(void) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    input();
    solve();
    
    return 0;
}

Measured memory_bytes: 6656