#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,a,b) for (int i = a; i <= b; i++)
#define per(i,a,b) for (int i = a; i >= b; i--)
#define pii pair<int,int>
#define fi first
#define se second
#define endl '\n'
#define pb push_back
#define all(v) (v).begin(), (v).end()
const int MAXN = 10;
const int INF = 1e18+10;
const int MOD = 1e9+7;
vector<pii> g[MAXN+5];
int dp[MAXN+5][(1<<MAXN)][(1<<MAXN)],n,T;
int dfs(int u, int blocked, int trail) {
if(u==T) return 0;
if(dp[u][blocked][trail] != -1) return dp[u][blocked][trail];
int best = INF;
dp[u][blocked][trail] = best;
auto mouse_move = [&](int bl, int tr) -> int {
int mx = -1;
for (auto &[v,j] : g[u]) {
if ((bl>>j&1) || (tr>>j&1)) continue;
mx = max(mx, dfs(v, bl, tr|(1<<j)));
}
return mx;
};
{
int mx = mouse_move(blocked, trail);
if (mx >= 0) best = min(best, mx);
}
rep(i,0,n-2) {
if(blocked>>i&1) continue;
int nb = blocked|(1<<i);
int mx = mouse_move(nb, trail);
if(mx!=-1) best = min(best,mx+1);
else best = min(best, dfs(u,nb,trail)+1);
}
rep(i, 0, n-2) {
if (!(trail>>i&1)) continue;
int nt = trail^(1<<i);
int mx = mouse_move(blocked, nt);
if (mx >= 0) best = min(best, mx + 1);
else best = min(best, dfs(u, blocked, nt) + 1);
}
dp[u][blocked][trail] = best;
return dp[u][blocked][trail];
}
void solve() {
int m; cin >> n >> T >> m;
rep(i,0,n-2) {
int u,v; cin >> u >> v;
g[u].pb({v,i});
g[v].pb({u,i});
}
memset(dp,-1,sizeof(dp));
cout << dfs(m,0,0) << endl;
}
int32_t main() {
ios_base::sync_with_stdio(0);cin.tie(nullptr);
int tt = 1;
//cin >> tt;
while(tt--) solve();
}
