const int Q = (int)2e5 + 7;

int n;
ll w[Q];

vi g[Q];
vll mxdst[Q];
ll ww[Q];

ll ans = -1;

void dfs1(int u)
{
    mxdst[u].resize(g[u].size());
    for (int i = 0; i < g[u].size(); ++i)
    {
        dfs1(g[u][i]);
        mxdst[u][i] = ww[g[u][i]];
    }
    ww[u] = w[u];
    for (int i = 0; i < g[u].size(); ++i)
    {
        ww[u] += mxdst[u][i];
    }
}

void dfs2(int u)
{
    for (int i = 0; i < g[u].size(); ++i)
    {
        dfs2(g[u][i]);
        ww[u] = max(ww[g[u][i]], ww[u]);
    }
}

void dfs3(int u)
{
    vll rs;
    for (int i = 0; i < g[u].size(); ++i)
    {
        dfs3(g[u][i]);
        rs.inb(ww[g[u][i]]);
    }
    if (rs.size() > 1)
    {
        sort(all(rs));
        ans = max(rs[rs.size() - 2] + rs[rs.size() - 1], ans);
    }
}

int solve()
{
    scanf("%d", &n);
    for (int i = 0; i < n; ++i)
        scanf("%lld", &w[i]);
    for (int i = 0; i < n - 1; ++i)
    {
        int x, y;
        scanf("%d %d", &x, &y);
        --x, --y;
        g[x].inb(y);
    }
    dfs1(0);
    dfs2(0);
    dfs3(0);
    if (ans == -1)
        puts("Impossible");
    else
        cout << ans;
    return 0;
}