#include <bits/stdc++.h>
using namespace std;
vector<int> g[100100];
int aa[100100];
long long dfs(int u, int fa) {
  vector<long long> tmp;
  for (int i = 0; i < g[u].size(); i++) {
    int v = g[u][i];
    if (v != fa && aa[v] - 1 >= 0) {
      aa[v] -= 1;
      tmp.push_back(dfs(v, u));
    }
  }
  sort(tmp.begin(), tmp.end());
  int cc = tmp.size();
  long long ans = 0;
  for (int i = aa[u]; i >= 1; i--) {
    if (cc > 0) {
      ans += tmp[--cc] + 2;
      aa[u]--;
    } else {
      break;
    }
  }
  for (int i = 0; i < g[u].size(); i++) {
    int v = g[u][i];
    if (v != fa) {
      int tmp = min(aa[u], aa[v]);
      ans += tmp * 2;
      aa[u] -= tmp;
      aa[v] -= tmp;
    }
  }
  return ans;
}
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &aa[i]);
  }
  for (int i = 0; i < n - 1; i++) {
    int u, v;
    scanf("%d%d", &u, &v);
    g[u].push_back(v);
    g[v].push_back(u);
  }
  int s;
  scanf("%d", &s);
  printf("%I64d\n", dfs(s, -1));
  return 0;
}
