CF77C

#include <bits/stdc++.h>

using ll = long long;
using namespace std;

int main()
{
    int n;
    cin >> n;
    vector<ll> a(n + 1);
    vector<vector<int>> G(n + 1);
    for (int i = 1; i <= n; i++) cin >> a[i];
    for (int i = 1; i < n; i++) {
        int u, v;
        cin >> u >> v;
        G[u].push_back(v), G[v].push_back(u);
    }
    int rt;
    cin >> rt;
    vector<ll> dp(n + 1);

    auto dfs = [&](auto dfs, int u, int fa) -> void {
        if (u != rt) a[u]--;
        vector<int> tmp;
        for (int v : G[u]) {
            if (v == fa) continue;
            dfs(dfs, v, u);
            tmp.push_back(v);
        }
        sort(tmp.begin(), tmp.end(), [&](int a, int b) { return dp[a] > dp[b]; });
        for (int i = 0; i < tmp.size() && a[u]; i++) {
            dp[u] += dp[tmp[i]] + 2;
            a[u]--;
        }
        if (a[u]) {
            for (int v : tmp) {
                if (a[v]) {
                    int t = min(a[v], a[u]);
                    dp[u] += 2 * t;
                    a[u] -= t;
                }
            }
        }
    };
    
    dfs(dfs, rt, 0);
    
    cout << dp[rt] << endl;
    return 0;

}