#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstdlib>
#include <vector>
#include <set>
#include <map>
#include <cassert>
#include <ctime>
#include <cmath>
#include <string>
#include <cstring>
#include <queue>
using namespace std;

#define f first
#define s second
#define mp make_pair
#define pb push_back
#define pii pair<int, int>
#define vi vector<int>
#define all(v) (v).begin(), (v).end()
#define forit(it,v) for (typeof(v.begin()) it = v.begin(); it != v.end(); ++it)
#define f0(a) memset(a, 0, sizeof(a))

const int maxn = 200000;
long long ans[maxn];
int n, k[maxn];
vi g[maxn];


void Dfs(int v, int parent) {

    vector<int> vec;

    ans[v] = 0;

    forit(it, g[v]) {
        int to = *it;

        if (to != parent) {
            k[to]--;
            Dfs(to, v);
            vec.pb(ans[to]);
        }
    }
    sort(all(vec));
    reverse(all(vec));

    long long sum = vec.size();
    for (int i = 0; k[v] > 0 && i < vec.size(); ++i) {
        ans[v] += vec[i] + 1;
        sum--;
        k[v]--;

    }

    forit(it, g[v]) {
        int to = *it;
        if (to != parent)
            sum += k[to];
    }

    if (v == 5) cerr << sum << endl;

    long long tmp = min(1ll * k[v], sum);

    ans[v] += 2*tmp;

    k[v] -= tmp;
}


int main() {
    #ifdef LOCAL
        freopen("in", "r", stdin);
        freopen("out", "w", stdout);
    #endif
    scanf("%d", &n);

    for (int i = 1; i <= n; ++i)
        scanf("%d", &k[i]);

    for (int i = 0; i < n - 1; ++i) {
        int u, v;
        scanf("%d%d", &u, &v);
        g[u].pb(v);
        g[v].pb(u);
    }

    int start;
    scanf("%d", &start);
    Dfs(start, -1);
    cout << ans[start]  << endl;
    return 0;
}