#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
const int N = 1000000;
const int INF = 0x3f3f3f3f;
int *ej[N], eo[N];
int pp[N], ss[N], ii[N], dp[N];
bool used[N];
void append(int i, int j) {
int o = eo[i]++;
if (!o)
ej[i] = (int *) malloc(sizeof *ej[i]);
else if (!(o & o - 1))
ej[i] = (int *) realloc(ej[i], (o << 1) * sizeof *ej[i]);
ej[i][o] = j;
}
void detach(int i, int j) {
for (int o = 0; o < eo[i]; o++)
if (ej[i][o] == j) {
swap(ej[i][o], ej[i][--eo[i]]);
return;
}
}
void dfs_pp(int p, int i) {
pp[i] = p;
for (int o = 0; o < eo[i]; o++) {
int j = ej[i][o];
if (j != p)
dfs_pp(i, j);
}
}
void dfs_ss(int p, int i, int s) {
ss[i] = s += eo[i] - 2;
for (int o = 0; o < eo[i]; o++) {
int j = ej[i][o];
if (j != p)
dfs_ss(i, j, s);
}
}
void dfs_dp(int i) {
int x = 0, y = 0;
for (int o = 0; o < eo[i]; o++) {
int j = ej[i][o];
dfs_dp(j);
int z = dp[j];
if (x < z)
y = x, x = z;
else if (y < z)
y = z;
}
dp[i] = y + eo[i];
}
bool check(int k, int x) {
for (int d = 0, h = k - 1; h >= 0; h--) {
int i = ii[h], s = h ? ss[ii[h - 1]] : 0;
int o = 0, o_ = eo[i];
while (o < o_ && dp[ej[i][o]] + o_ + s <= x)
o++;
if ((++d -= o_ - o) < 0 || (x -= o_ - o) < 0)
return false;
}
return true;
}
int main() {
ios_base::sync_with_stdio(false), cin.tie(NULL);
int n, s, t; cin >> n >> t >> s, s--, t--;
for (int h = 0; h < n - 1; h++) {
int i, j; cin >> i >> j, i--, j--;
append(i, j), append(j, i);
}
dfs_pp(-1, s);
if (s != t)
dfs_ss(t, pp[t], 0);
for (int i = 0; i < n; i++)
detach(i, pp[i]);
int k = 0;
for (int i = t; i != s; i = pp[i])
detach(ii[k++] = pp[i], i);
for (int h = 0; h < k; h++) {
int i = ii[h];
for (int o = 0; o < eo[i]; o++)
dfs_dp(ej[i][o]);
sort(ej[i], ej[i] + eo[i], [] (int j0, int j1) { return dp[j0] < dp[j1]; });
}
int lower = -1, upper = INF;
while (upper - lower > 1) {
int x = lower + upper >> 1;
if (check(k, x))
upper = x;
else
lower = x;
}
cout << upper << '\n';
return 0;
}
