#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double db;
typedef pair <int, int> pii;
typedef pair <ll, ll> pll;
#define fir first
#define sec second
typedef vector <int> vi;
typedef vector <ll> vl;
template <typename __Tp> void read(__Tp &x) {
int f = 0; x = 0; char ch = getchar();
for (; !isdigit(ch); ch = getchar()) if (ch == '-') f = 1;
for (; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + (ch ^ 48);
if (f) x = -x;
}
template <typename __Tp1, typename ...__Tp2> void read(__Tp1 &x, __Tp2 &...y) { read(x), read(y...); }
template <typename __Tp> void write(__Tp x) {
if (x < 0) putchar('-'), x = -x;
if (x > 9) write(x / 10);
putchar(x % 10 + 48);
}
void write(char ch) { putchar(ch); }
template <typename __Tp1, typename ...__Tp2> void write(__Tp1 x, __Tp2 ...y) { write(x), write(y...); }
const int maxn = 3e5 + 10;
int n, s, t;
vi e[maxn];
int vis[maxn], a[maxn], tot;
int dfs(int u) {
vis[u] = 1, a[++tot] = u;
if (u == t) return 1;
for (int v : e[u]) if (!vis[v] && dfs(v)) return 1;
vis[u] = 0, --tot;
return 0;
}
int f[maxn];
void dfs2(int u) {
vis[u] = 1; vi vec;
for (int v : e[u]) if (!vis[v]) dfs2(v), vec.push_back(f[v]);
sort(begin(vec), end(vec), greater <int> ());
f[u] = 0;
for (int i = 1; i <= (int) vec.size(); ++i) f[u] = max(f[u], vec[i - 1] + i);
}
pii query(int x) {
fill(vis, vis + n + 1, 0);
vis[a[x + 1]] = 1, dfs2(s);
vis[a[x + 1]] = 0, dfs2(t);
return pii(f[s], f[t]);
}
int ask(int x) {
if (x >= tot) return 1e9;
pii p = query(x);
return max(p.fir, p.sec);
}
int main() {
read(n, s, t);
for (int i = 1; i < n; ++i) {
int u, v; read(u, v);
e[u].push_back(v), e[v].push_back(u);
}
dfs(s);
int l = 1, r = tot - 1, mid, res = 1;
while (l <= r) {
mid = (l + r) >> 1;
pii p = query(mid);
if (p.fir <= p.sec) res = mid, l = mid + 1;
else r = mid - 1;
}
write(min(ask(res), ask(res + 1)), '\n');
return 0;
}
