#include <algorithm>
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cstdio>
using namespace std;
#define re register
struct edge {
    int to;
    edge *nx;
} e[2001000], *la[1001000], *cnt = e;
int f[1001000], q[1001000], fa[1001000], ll[1001000], rr[1001000], no[1001000], vl[1001000], d[1001000],
    xl[1001000], clnt, cmt;
inline void addE(re int a, re int b) {
    *++cnt = (edge) {
        b, la[a]
    };
    la[a] = cnt;
    *++cnt = (edge) {
        a, la[b]
    };
    la[b] = cnt;
}
void dfs(re int a, re int ff) {
    re int xx = 0, yy = 0;
    fa[a] = ff;

    for (re edge *i = la[a]; i; i = i->nx)
        if (i->to != ff) {
            dfs(i->to, a);

            if (f[i->to] > xx)
                yy = xx, xx = f[i->to];
            else if (f[i->to] > yy)
                yy = f[i->to];

            f[a]++;
            d[a]++;
        }

    f[a] += yy;
}
int qq[4001000], nn[4001000], lazy[4001000], l1, r1, m1, vis[1001000];
inline void pushup(re int i) {
    qq[i << 1] > qq[(i << 1) | 1] ? (qq[i] = qq[i << 1], nn[i] = nn[i << 1]) : (qq[i] = qq[(i << 1) | 1],
            nn[i] = nn[(i << 1) | 1]);
}
inline void pushdown(re int i) {
    lazy[i << 1] += lazy[i];
    lazy[(i << 1) | 1] += lazy[i];
    qq[i << 1] += lazy[i];
    qq[(i << 1) | 1] += lazy[i];
    lazy[i] = 0;
}
void build(re int i, re int l, re int r) {
    if (l == r) {
        qq[i] = vl[l];
        nn[i] = l;
        return;
    }

    re int mid = (l + r) >> 1;
    build(i << 1, l, mid);
    build((i << 1) | 1, mid + 1, r);
    pushup(i);
}
void add(re int i, re int l, re int r) {
    if (l >= l1 && r <= r1) {
        qq[i] += m1;
        lazy[i] += m1;
        return;
    }

    re int mid = (l + r) >> 1;

    if (lazy[i])
        pushdown(i);

    if (mid >= l1)
        add(i << 1, l, mid);

    if (mid < r1)
        add((i << 1) | 1, mid + 1, r);

    pushup(i);
}
int find(re int a) {
    return vis[a] == a ? a : vis[a] = find(vis[a]);
}
char B[1 << 26], *S = B;
int r() {
    for (; *S < '-'; S++)
        ;

    register int x = *S++ -'0';

    for (; *S >= '0'; x = (x << 3) + (x << 1) + *S++ -'0')
        ;

    return x;
}
int main() {
    fread(B, 1, 1 << 26, stdin);
    re int n = r(), t = r(), s = r(), x, y, z;
    re long long ans = 0;

    for (re int i = 1; i < n; i++)
        x = r(), y = r(), addE(x, y);

    dfs(t, 0);

    for (re int i = s; i != t; i = fa[i])
        ll[++clnt] = i;

    for (re int i = clnt; i; i--)
        xl[i] = d[ll[i]] - 1 + xl[i + 1], vis[i] = i;

    xl[1]++;

    for (re int i = 1; i <= clnt; i++) {
        for (re edge *i1 = la[ll[i]]; i1; i1 = i1->nx)
            if (i1->to != fa[ll[i]] && i1->to != ll[i - 1]) {
                no[++cmt] = i;
                vl[cmt] = xl[i] + f[i1->to];
            }

        rr[i] = cmt;
    }

    if (cmt == 0)
        puts("0"), exit(0);

    build(1, 1, cmt);

    for (re int i = 1; i <= cmt; i++) {
        x = nn[1];
        y = no[x];
        z = find(y);

        if (!z)
            printf("%lld", qq[1] + ans), exit(0);
        else {
            vis[z] = z - 1;
            ans++;
            l1 = r1 = x;
            m1 = -1 << 25;
            add(1, 1, cmt);
            l1 = 1, r1 = rr[y], m1 = -1;
            add(1, 1, cmt);
        }
    }

    printf("%lld", cmt);
}