#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <cmath>

#include <algorithm>

#include <assert.h>

#define IOS ios::sync_with_stdio(false)

using namespace std;

typedef long long int LL;

const LL inf = 1e16L;

#include <iostream>

#include <sstream>

#include <vector>

#include <set>

#include <map>

#include <queue>

#include <string>

const int maxn = 2e5 + ;

int a[maxn];

struct node {

    int u, v, w;

    int tonext;

}e[maxn << ];

int first[maxn];

int num;

void add(int u, int v) {

    ++num;

    e[num].u = u;

    e[num].v = v;

    e[num].tonext = first[u];

    first[u] = num;

}

bool vis[maxn];

LL dp[maxn];

LL mx[maxn];

void init(int cur) {

    dp[cur] = a[cur];

    mx[cur] = -inf;

    for (int i = first[cur]; i; i = e[i].tonext) {

        int v = e[i].v;

        if (vis[v]) continue;

        vis[v] = true;

        init(v);

        dp[cur] += dp[v];

        mx[cur] = max(mx[cur], mx[v]);

//        mx[cur] = max(mx[cur], dp[v]);

    }

    mx[cur] = max(dp[cur], mx[cur]);

}

LL ans = -inf;

void dfs(int cur, int fa) {

    LL mx1 = -inf, mx2 = -inf;

    int t = ;

    for (int i = first[cur]; i; i = e[i].tonext) {

        int v = e[i].v;

        if (fa == v) continue;

        dfs(v, cur);

        t++;

        if (mx1 <= mx[v]) {

            mx2 = mx1;

            mx1 = mx[v];

        } else if (mx2 <= mx[v]) {

            mx2 = mx[v];

        }

    }

    if (t ==  || t == ) return;

    ans = max(ans, mx1 + mx2);

}

void work() {

    int n;

    scanf("%d", &n);

    for (int i = ; i <= n; ++i) {

        scanf("%d", &a[i]);

    }

    for (int i = ; i <= n - ; ++i) {

        int u, v;

        scanf("%d%d", &u, &v);

        add(u, v);

        add(v, u);

    }

    vis[] = true;

    init();

//    printf("%d\n", dp[6]);

//    cout << mx[5] << endl;

    memset(vis, , sizeof vis);

    dfs(, );

    if (ans <= -inf) {

        cout << "Impossible" << endl;

    } else cout << ans << endl;

}

int main() {

#ifdef local

    freopen("data.txt", "r", stdin);

//    freopen("data.txt", "w", stdout);

#endif

    work();

    return ;

}