#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <string>
#include <cmath>
#include <cstdio>
#include <iomanip>
#include <fstream>
#include <cassert>
#include <cstring>
#include <unordered_set>
#include <unordered_map>
#include <numeric>
#include <ctime>
#include <bitset>
#include <complex>
#include <random>

using namespace std;

vector<int> used;

vector<vector<int>> g;

void dfs(int cur, int ch, vector<int> &h) {
    h[cur] = ch;
    used[cur] = 1;
    for (auto t : g[cur]) {
        if (!used[t]) {
            dfs(t, ch + 1, h);
        }
    }
}

int mys(int len) {
    return len - 1;
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    int n;
    int a, b;
    cin >> n >> a >> b;
    a--;
    b--;
    used.resize(n);
    g.resize(n);
    for (int i = 0; i + 1 < n; i++) {
        int u, v;
        cin >> u >> v;
        u--;
        v--;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    vector<int> ha(n);
    vector<int> hb(n);
    dfs(a, 0, ha);
    fill(used.begin(), used.end(), 0);
    dfs(b, 0, hb);
    int ans = 0;
    for (int i = 0; i < n; i++) {
        if (hb[i] > ha[i]) {
            ans = max(ans, ha[i] + mys(hb[i] - ha[i]));
        }
    }
    cout << ans << endl;
}