#include <bits/stdc++.h>

 using namespace std;

 const int MAXN = ;

 int n, a[MAXN], in[MAXN];

 vector<int> G[MAXN];

 double dp[MAXN][];

 bool vis[MAXN];

 void dfs(int now)

 {

     vis[now] = true;

     for (int i = ; i < G[now].size(); i++)

     {

         int tmp = G[now][i];

         if (!vis[tmp]) dfs(tmp);

         dp[now][] += max(dp[tmp][], dp[tmp][] + a[tmp]);

         dp[now][] += max(dp[tmp][], dp[tmp][] + (double)a[tmp] / 2.0);

     }

 }

 int main()

 {

     cin >> n;

     int u, v;

     for (int i = ; i <= n; i++) cin >> a[i];

     for (int i = ; i < n - ; i++) { cin >> u >> v; in[v]++; G[u].push_back(v); }

     int i = ;

     for (; i <= n; i++) if (!in[i]) break;

     dfs(i);

     printf("%.1f\n", max(dp[i][], dp[i][] + a[i]));

     return ;

 }