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

#define MAXN 300010

int N, a, b;
vector< int >colleg[MAXN];
bool visited[MAXN];
int scelta[MAXN];

int Dfs(int n, int verso) {
    visited[n]=true;
    if(n == verso) return scelta[n] = n;

    for(int i : colleg[n]) {
        if(visited[i]) continue;
        int tmp = Dfs(i, verso);
        if(tmp > -1) { scelta[n] = i; break; }
    }

    return scelta[n];
}

int Colora(int rad, pair<int, int>bloccato) {
    visited[rad]=true;
    vector< int >cosi;
    for(int i : colleg[rad]) {
        if( (make_pair(rad, i) ==bloccato) || (make_pair(i, rad)==bloccato) ) continue;
        if(visited[i]) continue;
        cosi.push_back(1 + Colora(i, bloccato));
    }

    sort(cosi.begin(), cosi.end());
    reverse(cosi.begin(), cosi.end());

    int ris=0;
    for(int i=0;i<(int)cosi.size();i++) ris=max(ris, i + cosi[i]);

    return ris;
}

pair<int, int> Controlla(pair<int, int>arco) {
    memset(visited, false, sizeof visited);
    int ans1 = Colora(a, arco);

    memset(visited, false, sizeof visited);
    int ans2 = Colora(b, arco);
    return {ans1, ans2};
}
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    cin >> N >> a >> b;
    int p, q;
    for(int i=0;i<N-1;i++) {
        cin>>p>>q;
        colleg[p].push_back(q);
        colleg[q].push_back(p);
    }

    memset(scelta, -1, sizeof scelta);
    Dfs(a, b);

    vector< pair<int, int> >v;
    int parto = a;
    while(parto != b) v.push_back({parto, scelta[parto]}), parto=scelta[parto];


    int ris = -1;
    int low=0, high=(int)v.size() - 1, mid=0;
    while(low <= high) {
        mid= (low+high)/2;
        pair<int, int> tmp_ans = Controlla(v[mid]);
        int tmp_a = tmp_ans.first, tmp_b = tmp_ans.second;
        pair<int, int> avanti = ((mid+1 >= (int)v.size()) ? make_pair(-1,-1) : Controlla(v[mid+1]));

        ris=(ris==-1) ? max(tmp_a, tmp_b) : min(ris, max(tmp_a, tmp_b));
        if(avanti.first==-1) break;

        if(max(tmp_a, tmp_b) > max(avanti.first, avanti.second)) low=mid+1;
        else if(max(tmp_a, tmp_b) > max(avanti.first, avanti.second)) high=mid-1;
        else {
            if(tmp_a < tmp_b) low=mid+1;
            else high=mid-1;

            /* if(tmp_a <= tmp_b) low=mid+1;
             * else high=mid-1;
            */
        }
    }

    cout << ris << endl;
    return EXIT_SUCCESS;
}