#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
 
#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
 
using namespace std;
using namespace __gnu_pbds;
 
typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;
 
typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;
 
typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;
 
template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;
 
#define FOR(i, a, b) for (int i=a; i<(b); i++)
#define F0R(i, a) for (int i=0; i<(a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= a; i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)
 
#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
 
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
 
const int MOD = 1000000007;
const ll INF = 1e18;
const int MX = 100001; //check the limits, dummy
 int n, l;
vector<vector<int>> adj;

int timer;
vector<int> tin, tout;
vector<vector<int>> up;
int depth[MX];
int maxDown[MX];
int dfs(int v, int p, int d)
{
    depth[v] = d;
    tin[v] = ++timer;
    up[v][0] = p;
    int maxVal = 0;
    for (int i = 1; i <= l; ++i)
        up[v][i] = up[up[v][i-1]][i-1];

    for (int u : adj[v]) {
        if (u != p)
            maxVal = max(maxVal, dfs(u, v, d+1));
    }

    tout[v] = ++timer;
    maxDown[v] = maxVal;
    return maxVal+1;
}

bool is_ancestor(int u, int v)
{
    return tin[u] <= tin[v] && tout[u] >= tout[v];
}

int lca(int u, int v)
{
    if (is_ancestor(u, v))
        return u;
    if (is_ancestor(v, u))
        return v;
    for (int i = l; i >= 0; --i) {
        if (!is_ancestor(up[u][i], v))
            u = up[u][i];
    }
    return up[u][0];
}

void preprocess(int root) {
    tin.resize(n);
    tout.resize(n);
    timer = 0;
    l = ceil(log2(n));
    up.assign(n, vector<int>(l + 1));
    dfs(root, root, 0);
}
int main() {
	ios_base::sync_with_stdio(0); cin.tie(0);    
	
    cin >> n;

    int U, V; cin >> U >> V; U--; V--;
    F0R(i, n) {
        vi blank; adj.pb(blank);
    }
    F0R(i, n-1) {
        int A, B; cin >> A >> B; A--; B--;
        adj[A].pb(B);
        adj[B].pb(A);
    }

    preprocess(0);
    int ans = 0;
    int lcaBoth = lca(U, V);
    F0R(i, n) {
        int lca1 = lca(i, U);
        int dist1 = depth[U] + depth[i] - 2*depth[lca1];
        if (is_ancestor(i, U)) {
            dist1 = depth[U] - depth[i];
        } else if (is_ancestor(U, i)) {
            dist1 = depth[i] - depth[U];
        }
        int lca2 = lca(i, V);
        int dist2 = depth[V] + depth[i] - 2*depth[lca2];
        if (is_ancestor(i, V)) {
            dist2 = depth[V] - depth[i];
        } else if (is_ancestor(V, i)) {
            dist2 = depth[i] - depth[V];
        }
        
        
        if (dist1 < dist2) {
            ans = max(ans, dist2-1);
        } else if (dist1 == dist2) {
            if (is_ancestor(lcaBoth, i) && (is_ancestor(i, U) || is_ancestor(i, V))) {
                ans = max(ans, dist2);
            }
        }
    }
    cout << ans << endl;
	
	return 0;
}
 
// read the question correctly (ll vs int)
// template by bqi343



Measured memory_bytes: 24304