#include <stdio.h>
#include <stdlib.h>
#define N 1000000
#define INF 0x3f3f3f3f
int min(int a, int b) { return a < b ? a : b; }
int max(int a, int b) { return a > b ? a : b; }
unsigned int X = 12345;
int rand_() {
return (X *= 3) >> 1;
}
int *ej[N], eo[N];
void append(int i, int j) {
int o = eo[i]++;
if (o >= 2 && (o & o - 1) == 0)
ej[i] = (int *) realloc(ej[i], o * 2 * sizeof *ej[i]);
ej[i][o] = j;
}
int dp[N];
void dfs_(int p, int i, int k) {
int o, mx1, mx2;
k += eo[i] - 1, mx1 = mx2 = k;
for (o = eo[i]; o--; ) {
int j = ej[i][o];
if (j != p) {
dfs_(i, j, k);
if (mx1 < dp[j])
mx2 = mx1, mx1 = dp[j];
else if (mx2 < dp[j])
mx2 = dp[j];
}
}
dp[i] = mx2;
}
void sort(int *ii, int l, int r) {
while (l < r) {
int i = l, j = l, k = r, i_ = ii[l + rand_() % (r - l)], tmp;
while (j < k)
if (dp[ii[j]] == dp[i_])
j++;
else if (dp[ii[j]] > dp[i_]) {
tmp = ii[i], ii[i] = ii[j], ii[j] = tmp;
i++, j++;
} else {
k--;
tmp = ii[j], ii[j] = ii[k], ii[k] = tmp;
}
sort(ii, l, i);
l = k;
}
}
int qu[N], gg[N], jj[N], n, m, l, ans; char path[N];
int solve() {
int g, g_, h;
for (g = 0, g_ = 0, h = 0; g <= l; g++) {
while (h < m && dp[jj[h]] + g_ <= ans) {
h++;
if (h < m && gg[jj[h]] != gg[jj[h - 1]])
g_ = g;
}
if (h == m)
return 1;
if (gg[jj[h]] < g || g == ans)
return 0;
h++;
if (h < m && gg[jj[h]] != gg[jj[h - 1]])
g_ = g + 1;
}
return 1;
}
int dfs(int p, int i, int t) {
int o;
qu[l++] = i, path[i] = 1;
if (i == t) {
int d, g, lower, upper;
l--, d = 0;
for (g = 0; g < l; g++) {
i = qu[g];
d += eo[i] - (g == 0 ? 1 : 2);
}
m = 0;
for (g = 0; g < l; g++) {
int m_;
i = qu[g];
m_ = m;
for (o = eo[i]; o--; ) {
int j = ej[i][o];
if (!path[j]) {
dfs_(i, j, d);
gg[j] = g, jj[m_] = j, m_++;
}
}
sort(jj, m, m_);
m = m_;
d -= eo[i] - (g == 0 ? 1 : 2);
}
lower = -1, upper = n;
while (upper - lower > 1) {
ans = (lower + upper) / 2;
if (solve())
upper = ans;
else
lower = ans;
}
ans = upper;
return 1;
}
for (o = eo[i]; o--; ) {
int j = ej[i][o];
if (j != p && dfs(i, j, t))
return 1;
}
l--, path[i] = 0;
return 0;
}
int main() {
int t, m, h, i, j;
scanf("%d%d%d", &n, &t, &m), t--, m--;
if (m == t) {
printf("0\n");
return 0;
}
for (i = 0; i < n; i++)
ej[i] = (int *) malloc(2 * sizeof *ej[i]);
for (h = 0; h < n - 1; h++) {
scanf("%d%d", &i, &j), i--, j--;
append(i, j), append(j, i);
}
dfs(-1, m, t);
printf("%d\n", ans);
return 0;
}
