#include <bits/stdc++.h>
#define FOR(i, l, r) for(int i=(l); i<=(r); ++i)
#define REP(i, l, r) for(int i=(l); i<(r); ++i)
#define FORD(i, r, l) for(int i=(r); i>=(l); --i)
#define REPD(i, r, l) for(int i=(r)-1; i>=(l); --i)
using namespace std;
const int N = 1e6 + 5;
int n, trap, mouse, cstep[N], deg[N], sdeg[N], p[N], avail[N];
vector<int> S[N], Q[N];
void Enter() {
cin >> n >> trap >> mouse;
REP(i, 1, n) {
int u, v;
cin >> u >> v;
S[u].push_back(v);
S[v].push_back(u);
}
}
void DFS(int u, int pa) {
if (u == mouse) avail[u] = 1;
p[u] = pa;
for(int v : S[u]) if (v != pa) {
DFS(v, u);
if (avail[v]) avail[u] = 1;
else {
Q[u].push_back(v);
++deg[u];
}
}
sort(Q[u].begin(), Q[u].end(), [] (int x, int y) { return cstep[x] > cstep[y]; });
if (deg[u] <= 1) cstep[u] = deg[u];
if (deg[u] > 1) cstep[u] = cstep[Q[u][1]] + deg[u];
}
void DFS2(int u) {
cstep[u] += sdeg[u];
for(int v : S[u]) if (v != p[u]) {
sdeg[v] = deg[u] + sdeg[u];
DFS2(v);
}
}
bool check(int x) {
int sl = 0, timer = 0;
for(int i = mouse; i != trap; i = p[i]) {
++timer;
int c_now = 0;
for(int v : Q[i]) if (cstep[v] + sl - c_now > x) ++sl, ++c_now;
if (sl > timer || sl > x) return false;
}
return true;
}
void Process() {
DFS(trap, 0);
deg[trap] = 0;
DFS2(trap);
int l = 0, r = n, mid;
while (l < r) {
mid = (l + r) / 2;
if (check(mid)) r = mid;
else l = mid + 1;
}
cout << l;
}
int main()
{
#define file "mousetrap"
if (fopen(file".inp", "r")) {
freopen(file".inp", "r", stdin);
freopen(file".out", "w", stdout);
}
ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
Enter();
Process();
}
