743D

//I_love_StarnyC 
#include<bits/stdc++.h>
#define ll long long
#define ld long double
#define f first
#define s second
#define pb push_back
#define fix  cout<<std::fixed;
#define mk make_pair
#define mod 1000000007
#define time  cerr<< '\n' << "Time elapsed :" << clock() * 1000.0 / CLOCKS_PER_SEC << " ms\n" ;
#define IOS        ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
vector <ll> v[200005];
ll vis[200005],val[200005],a[200005],b[200005];
ll m=-1e18;
void dfs(ll n)
{  ll g=-1e18;
    if(vis[n]==1)return ;
    vis[n]=1;
    vector <ll> d;
    for(auto i:v[n])
    {
        if(vis[i]==0){dfs(i);
        a[n-1]+=a[i-1];
        g=max(g,val[i-1]);
        d.pb(val[i-1]);
    }
}
    sort(d.rbegin(),d.rend());
    if(d.size()>=2)
   m=max(m,d[0]+d[1]);
    val[n-1]=max(g,a[n-1]);
}
 
 
int main()
{  
    ll n;cin>>n;
    for(int i=0;i<n;i++)
        {cin>>a[i];b[i]=a[i];}
 
        for(int i=0;i<n-1;i++)
        {
            int a,b;cin>>a>>b;
            v[a].pb(b);
            v[b].pb(a);
        }
 
        dfs(1);
        if(m==-1e18)cout<<"Impossible";
        else
            
        cout<<m;
 
}