#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
const long long N = 2e5 + 10;
long long a[N];
vector<long long> v[N];
long long dp[N][2];
long long mx = -1e18;
void dfs(long long s, long long par = 0) {
  for (long long it : v[s]) {
    if (it == par) continue;
    dfs(it, s);
  }
  dp[s][0] = a[s];
  vector<long long> ans;
  for (long long it : v[s]) {
    if (it == par) continue;
    dp[s][0] += dp[it][0];
    ans.push_back(dp[it][1]);
  }
  dp[s][1] = dp[s][0];
  sort(ans.rbegin(), ans.rend());
  if (ans.size() > 1) {
    mx = max(mx, ans[0] + ans[1]);
  }
  for (long long it : v[s]) {
    if (it == par) continue;
    dp[s][1] = max(dp[s][1], dp[it][1]);
  }
}
void solve(long long cas) {
  long long n;
  cin >> n;
  for (long long i = 1; i <= n; ++i) cin >> a[i];
  for (long long i = 1; i < n; ++i) {
    long long x, y;
    cin >> x >> y;
    v[x].push_back(y);
    v[y].push_back(x);
  }
  dfs(1);
  if (mx == -1e18)
    cout << "Impossible\n";
  else
    cout << mx << '\n';
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  long long t = 1;
  long long cas = 0;
  while (t--) solve(++cas);
}
