#include <bits/stdc++.h>
using namespace std;
const long long INF(0x3f3f3f3f3f3f3f3fll);
const int inf(0x3f3f3f3f);
template <typename T>
void read(T &res) {
  bool flag = false;
  char ch;
  while (!isdigit(ch = getchar())) (ch == '-') && (flag = true);
  for (res = ch - 48; isdigit(ch = getchar());
       res = (res << 1) + (res << 3) + ch - 48)
    ;
  flag && (res = -res);
}
template <typename T>
void Out(T x) {
  if (x < 0) putchar('-'), x = -x;
  if (x > 9) Out(x / 10);
  putchar(x % 10 + '0');
}
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long lcm(long long a, long long b) { return a * b / gcd(a, b); }
long long pow_mod(long long x, long long n, long long mod) {
  long long res = 1;
  while (n) {
    if (n & 1) res = res * x % mod;
    x = x * x % mod;
    n >>= 1;
  }
  return res;
}
long long fact_pow(long long n, long long p) {
  long long res = 0;
  while (n) {
    n /= p;
    res += n;
  }
  return res;
}
long long mult(long long a, long long b, long long p) {
  a %= p;
  b %= p;
  long long r = 0, v = a;
  while (b) {
    if (b & 1) {
      r += v;
      if (r > p) r -= p;
    }
    v <<= 1;
    if (v > p) v -= p;
    b >>= 1;
  }
  return r;
}
long long quick_pow(long long a, long long b, long long p) {
  long long r = 1, v = a % p;
  while (b) {
    if (b & 1) r = mult(r, v, p);
    v = mult(v, v, p);
    b >>= 1;
  }
  return r;
}
bool CH(long long a, long long n, long long x, long long t) {
  long long r = quick_pow(a, x, n);
  long long z = r;
  for (long long i = 1; i <= t; i++) {
    r = mult(r, r, n);
    if (r == 1 && z != 1 && z != n - 1) return true;
    z = r;
  }
  return r != 1;
}
bool Miller_Rabin(long long n) {
  if (n < 2) return false;
  if (n == 2) return true;
  if (!(n & 1)) return false;
  long long x = n - 1, t = 0;
  while (!(x & 1)) {
    x >>= 1;
    t++;
  }
  srand(time(NULL));
  long long o = 8;
  for (long long i = 0; i < o; i++) {
    long long a = rand() % (n - 1) + 1;
    if (CH(a, n, x, t)) return false;
  }
  return true;
}
const long long N = 1e6 + 10;
struct node {
  long long nex, to, w;
} edge[N];
long long head[N], tot, sum[N], ans, dp[N], n, w[N];
void add_edge(long long u, long long v, long long w) {
  edge[++tot].nex = head[u];
  edge[tot].to = v;
  edge[tot].w = w;
  head[u] = tot;
}
void dfs(long long u, long long fa) {
  sum[u] = w[u];
  for (long long i = head[u]; i; i = edge[i].nex) {
    long long v = edge[i].to;
    if (v == fa) continue;
    dfs(v, u);
    sum[u] += sum[v];
  }
}
void dfs1(long long u, long long fa) {
  for (long long i = head[u]; i; i = edge[i].nex) {
    long long v = edge[i].to;
    if (v == fa) continue;
    dfs1(v, u);
    if (dp[u] != -INF && dp[v] != -INF) ans = max(ans, dp[u] + dp[v]);
    dp[u] = max(dp[u], dp[v]);
  }
  dp[u] = max(dp[u], sum[u]);
}
signed main() {
  std::ios::sync_with_stdio(false);
  cin >> n;
  for (long long i = 1; i <= n; i++) cin >> w[i];
  for (long long i = 2; i <= n; i++) {
    long long u, v;
    cin >> u >> v;
    add_edge(u, v, 1);
    add_edge(v, u, 1);
  }
  ans = -INF;
  for (long long i = 1; i <= n; i++) dp[i] = -INF;
  dfs(1, -1);
  dfs1(1, -1);
  if (ans <= -INF)
    cout << "Impossible\n";
  else
    cout << ans << '\n';
  return 0;
}
