#include <bits/stdc++.h>
#define int long long
#define ll long long
using ull = unsigned long long;
using namespace std;
const int INF = 1e10;
const int MOD = 1e9 + 7;
#define dump(x)                             \
    if (dbg) {                              \
        cerr << #x << " = " << (x) << endl; \
    }
#define overload4(_1, _2, _3, _4, name, ...) name
#define FOR1(n) for (ll i = 0; i < (n); ++i)
#define FOR2(i, n) for (ll i = 0; i < (n); ++i)
#define FOR3(i, a, b) for (ll i = (a); i < (b); ++i)
#define FOR4(i, a, b, c) for (ll i = (a); i < (b); i += (c))
#define FOR(...) overload4(__VA_ARGS__, FOR4, FOR3, FOR2, FOR1)(__VA_ARGS__)
#define FORR(i, a, b) for (int i = (a); i <= (b); ++i)
template <class T>
bool chmin(T& a, const T& b) {
    if (a > b) {
        a = b;
        return 1;
    } else
        return 0;
}
template <class T>
bool chmax(T& a, const T& b) {
    if (a < b) {
        a = b;
        return 1;
    } else
        return 0;
}
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define all(v) (v).begin(), (v).end()
#define SZ(x) ((int)(x).size())
#define P pair<int, int>
#define V vector<int>
//#define S set<int>
#define itn int
bool dbg = false;

struct LowestCommonAncestor {
    int n, h;
    vector<vector<int>> G, par;
    vector<int> dep;
    LowestCommonAncestor() {}
    LowestCommonAncestor(int n) : n(n), G(n), dep(n) {
        h = 1;
        while ((1 << h) <= n)
            h++;
        par.assign(h, vector<int>(n, -1));
    }

    void add_edge(int u, int v) {
        G[u].emplace_back(v);
        G[v].emplace_back(u);
    }

    void dfs(int v, int p, int d) {
        par[0][v] = p;
        dep[v] = d;
        for (int u : G[v])
            if (u != p)
                dfs(u, v, d + 1);
    }

    void build(int r = 0) {
        dfs(r, -1, 0);
        for (int k = 0; k + 1 < h; k++)
            for (int v = 0; v < n; v++)
                if (~par[k][v])
                    par[k + 1][v] = par[k][par[k][v]];
    }

    int lca(int u, int v) {
        if (dep[u] > dep[v])
            swap(u, v);
        for (int k = 0; k < h; k++)
            if ((dep[v] - dep[u]) >> k & 1)
                v = par[k][v];

        if (u == v)
            return u;

        for (int k = h - 1; k >= 0; k--)
            if (par[k][u] != par[k][v])
                u = par[k][u], v = par[k][v];

        return par[0][u];
    }

    int distance(int u, int v) {
        return dep[u] + dep[v] - dep[lca(u, v)] * 2;
    }
};
//それHL分解じゃなくていいの？

int N;
int u, v;
int A[101010], B[101010];

void solve() {
    LowestCommonAncestor lca(N);
    FOR(N - 1)
    lca.add_edge(A[i], B[i]);
    lca.build(v);
    int d;
    int M = 0;
    if (lca.distance(u, v) % 2 == 0) {

        d = lca.distance(u, v) / 2 - 1;
        FOR(N) {
            if (lca.distance(u, i) == d && lca.dep[i] + d == lca.dep[u]) {
                M = i;
                break;
            }
        }
        //cout << M << endl;
        int ans = 0;
        FOR(N) {
            if (lca.lca(i, M) == M) {
                chmax(ans, lca.distance(i, M));
            }
        }
        ans -= 1;
        ans += lca.distance(v, M);
        cout << ans << endl;

    } else {
        d = lca.distance(u, v) / 2;
        FOR(N) {
            if (lca.distance(u, i) == d && lca.dep[i] + d == lca.dep[u]) {
                M = i;
                break;
            }
        }
        //cout << M << endl;
        int ans = 0;
        FOR(N) {
            if (lca.lca(i, M) == M) {
                chmax(ans, lca.distance(i, M));
            }
        }
        ans -= 1;
        ans += lca.distance(v, M);
        cout << ans << endl;
    }
}

signed main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cin >> N >> u >> v;
    u--;
    v--;
    FOR(N - 1) {
        cin >> A[i] >> B[i];
        A[i]--;
        B[i]--;
    }
    solve();
    return 0;
}


Measured memory_bytes: 24560