#include <bits/stdc++.h>
#define int long long
#define ll long long
#define fi first
#define se second
#define pb push_back
#define pii pair<int, int>
#define sz(v) (int)(v).size()
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define foru(i,a,b) for(int i = (a); i <= (b); ++i)
#define ford(i,a,b) for(int i = (a); i >= (b); --i)
template<typename T> bool maximize(T &res, const T &val) { if (res < val) { res = val; return true; } return false; }
template<typename T> bool minimize(T &res, const T &val) { if (val < res) { res = val; return true; } return false; }
using namespace std;
const int N = 1e6 + 5;
const int inf = 1e18;
vector <int> adj[N];
int n, t, m, dp[N], par[N];
void dfs(int pos, int prev)
{
if (prev != -1)
adj[pos].erase(find(adj[pos].begin(), adj[pos].end(), prev));
int mx = 0, sc = 0;
for (auto x : adj[pos])
{
par[x] = pos;
dfs(x, pos);
if (dp[x] >= mx) sc = mx, mx = dp[x];
else sc = max(sc, dp[x]);
}
dp[pos] = sc + adj[pos].size();
}
int32_t main()
{
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
cin>>n>>t>>m;
t--; m--;
for (int i=0; i<n-1; i++)
{
int a, b; cin>>a>>b;
a--; b--;
adj[a].push_back(b);
adj[b].push_back(a);
}
dfs(t, -1);
int ttld = 0, l = -1, r = n;
while (l + 1 < r)
{
int mid = (l+r)/2, ok = 1, dif = 0, prev = -1, canuse = 0, cur = m, used = 0;
while (cur != t)
{
canuse++; int tmp = 0;
for (auto x:adj[cur])
{
if (x==prev) continue;
if (dp[x] - dif + used > mid)
{
if (!canuse) ok = 0;
else canuse--, tmp++;
}
}
used += tmp, dif += adj[cur].size() - (prev != -1);
prev = cur, cur = par[cur];
}
ttld = dif;
if (ok) r = mid;
else l = mid;
}
cout<<r + ttld;
return 0;
}
