#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
const long long INF = 1e18 + 7;
const long long N = 2e5 + 5;
const long double EPS = 1e-3;
const long long MOD = 998244353;
long long a[N], ans[N], s;
vector<vector<long long>> g(N);
vector<bool> added(N);
void calc(long long v, long long p) {
  set<pair<long long, long long>, greater<pair<long long, long long>>> add;
  for (auto &i : g[v]) {
    if (i == p) continue;
    calc(i, v);
    add.insert({ans[i], i});
  }
  for (auto &i : add) {
    auto [t, first] = i;
    if (a[v] > 1 && a[first]) {
      added[first] = 1;
      ans[v] += t + 2;
      a[v]--;
      a[first]--;
    }
  }
  for (auto &i : g[v]) {
    if (i == p) continue;
    long long mn = max(min(a[v] - 1, a[i]), 0ll);
    a[v] -= mn;
    a[i] -= mn;
    ans[v] += mn * 2;
  }
}
void solve() {
  long long n;
  cin >> n;
  for (long long i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (long long i = 0; i + 1 < n; i++) {
    long long u, v;
    cin >> u >> v;
    --u, --v;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  long long s;
  cin >> s;
  --s;
  calc(s, -1);
  vector<pair<long long, long long>> add;
  for (auto &i : g[s]) {
    if (a[i] && !added[i]) {
      add.emplace_back(ans[i], i);
    }
  }
  sort(add.rbegin(), add.rend());
  for (auto &i : add) {
    auto [q, idx] = i;
    if (a[s]) {
      a[s]--;
      a[idx]--;
      ans[s] += ans[idx] + 2;
    }
  }
  for (auto &i : g[s]) {
    long long minus = min(a[i], a[s]);
    ans[s] += minus * 2;
    a[s] -= minus;
  }
  cout << ans[s] << '\n';
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  long long tests = 1;
  while (tests-- > 0) {
    solve();
  }
  return 0;
}
