#include <bits/stdc++.h>
using namespace std;
struct node {
  int y, ne;
} e[200010];
int f[100005], h[100005], np, vis[100005], s;
long long dp[100005], r[100005];
long long saya(int n) {
  if (dp[n] != -1) return dp[n];
  vis[n] = 1;
  long long ans = 0, res = 0;
  vector<long long> v;
  for (int i = h[n]; i; i = e[i].ne) {
    if (!vis[e[i].y]) {
      v.push_back(saya(e[i].y));
      res += r[e[i].y];
    }
  }
  sort(v.begin(), v.end());
  int j = f[n] - 1;
  if (n == s) j++;
  for (int i = v.size() - 1; i >= 0 && j > 0; i--, j--) {
    ans += v[i];
  }
  if (j != 0) {
    ans += 2 * min(res, 1ll * j);
    r[n] = j - min(res, 1ll * j);
  }
  ans += f[n] - j;
  return dp[n] = ans;
}
void add(int a, int b) {
  e[np].y = b;
  e[np].ne = h[a];
  h[a] = np;
  np++;
}
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", &f[i]);
  np = 1;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    scanf("%d%d", &a, &b);
    add(a, b);
    add(b, a);
  }
  scanf("%d", &s);
  memset(dp, -1, sizeof(dp));
  printf("%I64d\n", saya(s));
}
