Generous sponsors of the olympiad in which Chloe and Vladik took part allowed all the participants to choose a prize for them on their own. Christmas is coming, so sponsors decided to decorate the Christmas tree with their prizes. 

They took n prizes for the contestants and wrote on each of them a unique id (integer from 1 to n). A gift i is characterized by integer ai — pleasantness of the gift. The pleasantness of the gift can be positive, negative or zero. Sponsors placed the gift 1 on the top of the tree. All the other gifts hung on a rope tied to some other gift so that each gift hung on the first gift, possibly with a sequence of ropes and another gifts. Formally, the gifts formed a rooted tree with n vertices.

The prize-giving procedure goes in the following way: the participants come to the tree one after another, choose any of the remaining gifts and cut the rope this prize hang on. Note that all the ropes which were used to hang other prizes on the chosen one are not cut. So the contestant gets the chosen gift as well as the all the gifts that hang on it, possibly with a sequence of ropes and another gifts.

Our friends, Chloe and Vladik, shared the first place on the olympiad and they will choose prizes at the same time! To keep themselves from fighting, they decided to choose two different gifts so that the sets of the gifts that hang on them with a sequence of ropes and another gifts don't intersect. In other words, there shouldn't be any gift that hang both on the gift chosen by Chloe and on the gift chosen by Vladik. From all of the possible variants they will choose such pair of prizes that the sum of pleasantness of all the gifts that they will take after cutting the ropes is as large as possible.

Print the maximum sum of pleasantness that Vladik and Chloe can get. If it is impossible for them to choose the gifts without fighting, print Impossible.

Input

The first line contains a single integer n (1 ≤ n ≤ 2·105) — the number of gifts.

The next line contains n integers a1, a2, ..., an ( - 109 ≤ ai ≤ 109) — the pleasantness of the gifts.

The next (n - 1) lines contain two numbers each. The i-th of these lines contains integers ui and vi (1 ≤ ui, vi ≤ n, ui ≠ vi) — the description of the tree's edges. It means that gifts with numbers ui and vi are connected to each other with a rope. The gifts' ids in the description of the ropes can be given in arbirtary order: vi hangs on ui or ui hangs on vi. 

It is guaranteed that all the gifts hang on the first gift, possibly with a sequence of ropes and another gifts.

Output

If it is possible for Chloe and Vladik to choose prizes without fighting, print single integer — the maximum possible sum of pleasantness they can get together.

Otherwise print Impossible.

Examples

Input

8
0 5 -1 4 3 2 6 5
1 2
2 4
2 5
1 3
3 6
6 7
6 8


Output

25

Input

4
1 -5 1 1
1 2
1 4
2 3


Output

2

Input

1
-1


Output

Impossible
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.ObjectInputStream.GetField;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.InputMismatchException;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Scanner;
import java.util.Stack;
import java.util.StringTokenizer;
import java.util.TreeMap;
import java.util.TreeSet;


	public class Q1 {
		
		static long MOD = 1000000007;
		static boolean b[], b1[], check;
		static ArrayList<Integer>[] amp, pa;
		static ArrayList<Graph>[] amp1;
		static int left[],s[],right[],sum[],dist[],cnt[],start[],color[],parent[],prime[],size[];
		static long ans = 0,k;
		static int p = 0;
		static FasterScanner sc = new FasterScanner(System.in);
		static Queue<Pair> queue = new LinkedList<>();
		static PriorityQueue<Integer> pq;
		static BufferedWriter log;
		static HashSet<Integer>[] hs;
		static HashMap<String,Integer> hm;
		static HashMap<Pair,Integer> hmp;
		static Stack<Integer> st;
		static Pair prr[];
		static long parent1[],parent2[],size1[],size2[],arr1[],SUM[],MAX[],arr[];
		public static void main(String[] args) throws Exception {
	    	new Thread(null, new Runnable() {
				public void run() {
					try {
						soln();
					} catch (Exception e) {
						System.out.println(e);
					}
				}
			}, "1", 1 << 26).start();
		}
		static long dp[];
		static int n;
		public static void soln() throws IOException {
			//FasterScanner in = new FasterScanner(new FileInputStream("C:\\Users\\Admin\\Desktop\\QCL.txt"));
			//PrintWriter out = new PrintWriter("C:\\Users\\Admin\\Desktop\\A1.txt");
			log = new BufferedWriter(new OutputStreamWriter(System.out));
			n = sc.nextInt();
			arr = sc.nextLongArray(n);
			amp = (ArrayList<Integer>[]) new ArrayList[n];
			for(int i =0 ;i< n; i++) amp[i] = new ArrayList<>();
			SUM = new long[n];
			MAX = new long[n];
			parent = new int[n];
			ans = Long.MIN_VALUE;
			b = new boolean[n];
			buildGraph(n-1);
			dfs(0);
			if(!check && amp[0].size()<=1) {
				System.out.println("Impossible");
				return;
			}
			/*ArrayList<Long> temp1 = new ArrayList<>();
			int temp = 0;
			while((amp[temp].size()<=2 && temp!=0) || (amp[temp].size()<=1 && temp==0)){
				if(amp[temp].get(0)!=parent[temp]) temp = amp[temp].get(0);
				else temp = amp[temp].get(1);
				//System.out.println(amp[temp].size()+" "+temp);
			}
			//System.out.println(temp);
			//for(long i : SUM) System.out.print(i+" ");System.out.println();
			//for(long i : MAX) System.out.print(i+" ");System.out.println();
			//System.out.println(amp[temp]);
			for(int i:amp[temp]) if(i!=parent[temp]) temp1.add(MAX[i]);
			Collections.sort(temp1,Collections.reverseOrder());*/
			//ans = Math.max(ans), temp1.get(0)+temp1.get(1));
			System.out.println(ans);
			log.close();
			//out.close();
		}

		static void dfs(int x){
			long sum = 0, max = Long.MIN_VALUE;
			b[x] = true;
			//System.out.println(amp[x].size());
			PriorityQueue<Long> pq=new PriorityQueue<>(10,Collections.reverseOrder());
			if(amp[x].size()>2) check = true;
			for(int i:amp[x]){
				if(!b[i]){
					parent[i] = x;
					dfs(i);
					sum+=SUM[i];
					max = Math.max(max, MAX[i]);
					pq.add(MAX[i]);
				}
			}
			SUM[x] = sum+arr[x];
			MAX[x] = Math.max(SUM[x],max);
			if(pq.size()>=2){
				ans = Math.max(ans, pq.poll()+pq.poll());
			}
			
		}
		public static void buildWeightedGraph(int n){
			for(int i =0 ;i< n; i++){
				int x = sc.nextInt(), y = sc.nextInt(), w = sc.nextInt();
				amp1[--x].add(new Graph(--y,w));
				amp1[y].add(new Graph(x,w));
			}
		}
		
		public static void seive(int n){
			b = new boolean[(n+1)];
			Arrays.fill(b, true);
			b[1] = false;
			for(int i = 2;i*i<=n;i++){
				if(b[i]){
					for(int p = 2*i;p<=n;p+=i){
						b[p] = false;
						prime[p] = i;
					}
				}
			}
			for(int i = 2;i<=n;i++){
				if(b[i]) prime[i] = i;
			}
		}
		
		static class Graph{
			int vertex;
			int weight;
			Graph(int v, int w){
				vertex = v;
				weight = w;
			}
		}
		static class Pair implements Comparable<Pair> {
			int u;
			int v;
			int i;
			public Pair(int u, int v, int i) {
				this.u = u;
				this.v = v;
				this.i = i;
			}
			
			public int hashCode() {
				int hu = (int) (u ^ (u >>> 32));
				int hv = (int) (v ^ (v >>> 32));
				return 31*hu + hv;
			}
	 
			public boolean equals(Object o) {
				Pair other = (Pair) o;
				return ((u == other.u && v == other.v));
			}
	 
			public int compareTo(Pair other) {
				return Long.compare(u, other.u); //!= 0 ? (Long.compare(u, other.u))// : (Long.compare(v, other.v));
				//return Long.compare(w, other.w);
			}
			
			public String toString() {
				return "[u=" + u + ", v=" + v + "]";
			}
		}
		
		public static void buildGraph(int n){
			for(int i =0;i<n;i++){
				int x = sc.nextInt()-1, y = sc.nextInt()-1;
				amp[x].add(y);
				amp[y].add(x);
			}
		}
		
		
		public static int getParent(long x){
			while(parent[(int) x]!=x){
				parent[ (int) x] = parent[(int) parent[ (int) x]];
				x = parent[ (int) x];
			}
			return (int) x;
		}
		static long min(long a, long b, long c){
			if(a<b && a<c) return a;
			if(b<c) return b;
			return c;
		}
		/*
		
		static class Pair3{
			int x, y ,z;
			Pair3(int x, int y, int z){
				this.x = x;
				this.y = y;
				this.z = z;
			}
		}*/
		static int sum(int n){
			String str = Integer.toString(n);
			int cnt = 0;
			for(int i = 0; i< str.length();i++){
				cnt+=(str.charAt(i)-'0');
			}
			return cnt;
		}
		 static void KMPSearch(String pat, String txt)
		    {
		        int M = pat.length();
		        int N = txt.length();
		 
		        // create lps[] that will hold the longest
		        // prefix suffix values for pattern
		        int lps[] = new int[M];
		        int j = 0;  // index for pat[]
		 
		        // Preprocess the pattern (calculate lps[]
		        // array)
		        computeLPSArray(pat,M,lps);
		 
		        int i = 0;  // index for txt[]
		        while (i < N)
		        {
		            if (pat.charAt(j) == txt.charAt(i))
		            {
		                j++;
		                i++;
		            }
		            if (j == M)
		            {
		               // parent.add((i-j));
		                j = lps[j-1];
		            }
		 
		            // mismatch after j matches
		            else if (i < N && pat.charAt(j) != txt.charAt(i))
		            {
		                // Do not match lps[0..lps[j-1]] characters,
		                // they will match anyway
		                if (j != 0)
		                    j = lps[j-1];
		                else
		                    i = i+1;
		            }
		        }
		    }
		 static void computeLPSArray(String pat, int M, int lps[])
		    {
		        // length of the previous longest prefix suffix
		        int len = 0;
		        int i = 1;
		        lps[0] = 0;  // lps[0] is always 0
		 
		        // the loop calculates lps[i] for i = 1 to M-1
		        while (i < M)
		        {
		            if (pat.charAt(i) == pat.charAt(len))
		            {
		                len++;
		                lps[i] = len;
		                i++;
		            }
		            else  // (pat[i] != pat[len])
		            {
		                // This is tricky. Consider the example.
		                // AAACAAAA and i = 7. The idea is similar 
		                // to search step.
		                if (len != 0)
		                {
		                    len = lps[len-1];
		 
		                    // Also, note that we do not increment
		                    // i here
		                }
		                else  // if (len == 0)
		                {
		                    lps[i] = len;
		                    i++;
		                }
		            }
		        }
		    }
		private static void permutation(String prefix, String str) {
		    int n = str.length();
		    if (n == 0); //hs.add(prefix);
		    else {
		        for (int i = 0; i < n; i++)
		            permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i+1, n));
		    }
		}
		public static char give(char c1,char c2){
			if(c1!='a' && c2!='a') return 'a';
			if(c1!='b' && c2!='b') return 'b';
			return 'c';
		}
		
		public static void buildTree(int n){
			int arr[] = sc.nextIntArray(n);
			for(int i = 0;i<n;i++){
				int x = arr[i]-1;
				amp[i+1].add(x);
				amp[x].add(i+1);
			}
		}
		
		static class SegmentTree {
			boolean st[];
			boolean lazy[];

			SegmentTree(int n) {
				int size = 4 * n;
				st = new boolean[size];
				Arrays.fill(st, true);
				lazy = new boolean[size];
				Arrays.fill(lazy, true);
				//build(0, n - 1, 1);
			}
			
			/*long[] build(int ss, int se, int si) {
				if (ss == se) {
					st[si][0] = 1;
					st[si][1] = 1;
					st[si][2] = 1;
					return st[si];
				}
				int mid = (ss + se) / 2;
				long a1[] = build(ss, mid, si * 2), a2[] = build(mid + 1, se,
						si * 2 + 1);
				long ans[] = new long[3];
				if (arr[mid] < arr[mid + 1]) {
					ans[1] = Math.max(a2[1], Math.max(a1[1], a1[2] + a2[0]));
					if (a1[1] == (mid - ss + 1))
						ans[0] = ans[1];
					else
						ans[0] = a1[0];
					if (a2[2] == (se - mid))
						ans[2] = ans[1];
					else
						ans[2] = a2[2];
				} else {
					ans[1] = Math.max(a1[1], a2[1]);
					ans[0] = a1[0];
					ans[2] = a2[2];
				}
				st[si] = ans;
				return st[si];
			}*/

			void update(int si, int ss, int se, int idx, long x) {
				if (ss == se) {
					//arr[idx] += val;
					st[si]=false;
				} 
				else {
					int mid = (ss + se) / 2;
					if(ss <= idx && idx <= mid)
			        {
			             update(2*si, ss, mid, idx, x);
			        }
			        else
			        { update(2*si+1, mid+1, se, idx, x);
			        }
					st[si] = st[2*si]|st[2*si+1];
				}
			}
			/*boolean get(int qs, int qe, int ss, int se, int si){
				if(qs>se || qe<ss) return 0;
				if (qs <= ss && qe >= se) {
					return st[si];
				}
				int mid = (ss+se)/2;
				return get(qs, qe, ss, mid, si * 2)+get(qs, qe, mid + 1, se, si * 2 + 1);
			}*/
			void updateRange(int node, int start, int end, int l, int r, boolean val)
			{
			    if(!lazy[node])
			    { 
			        // This node needs to be updated
			        st[node] = lazy[node];    // Update it
			        if(start != end)
			        {
			            lazy[node*2] = lazy[node];                  // Mark child as lazy
			            lazy[node*2+1] = lazy[node];                // Mark child as lazy
			        }
			        lazy[node] = true;                                  // Reset it
			    }
			    if(start > end || start > r || end < l)              // Current segment is not within range [l, r]
			        return;
			    if(start >= l && end <= r)
			    {
			        // Segment is fully within range
			        st[node] =  val;
			        if(start != end)
			        {
			            // Not leaf node
			            lazy[node*2] = val;
			            lazy[node*2+1] = val;
			        }
			        return;
			    }
			    int mid = (start + end) / 2;
			    updateRange(node*2, start, mid, l, r, val);        // Updating left child
			    updateRange(node*2 + 1, mid + 1, end, l, r, val);   // Updating right child
			    st[node] = st[node*2] | st[node*2+1];        // Updating root with max value 
			}

			boolean queryRange(int node, int start, int end, int l, int r)
			{
			    if(start > end || start > r || end < l)
			        return false;         // Out of range
			    if(!lazy[node])
			    {
			        // This node needs to be updated
			        st[node] = lazy[node];            // Update it
			        if(start != end)
			        {
			            lazy[node*2] = lazy[node];         // Mark child as lazy
			            lazy[node*2+1] = lazy[node];    // Mark child as lazy
			        }
			        lazy[node] = true;                 // Reset it
			    }
			    if(start >= l && end <= r)             // Current segment is totally within range [l, r]
			        return  st[node];
			    int mid = (start + end) / 2;
			    boolean p1 = queryRange(node*2, start, mid, l, r);         // Query left child
			    boolean p2 = queryRange(node*2 + 1, mid + 1, end, l, r); // Query right child
			    return (p1 | p2);
			}
			void print() {
				for (int i = 0; i < st.length; i++) {
					System.out.print(st[i]+" ");
				}
				System.out.println();
			}
		}
		static int convert(int x){
			int cnt = 0;
			String str = Integer.toBinaryString(x);
			//System.out.println(str);
			for(int i = 0;i<str.length();i++){
				if(str.charAt(i)=='1'){
					cnt++;
				}
			}
			int ans = (int) Math.pow(3, 6-cnt);
			return ans;
		}
		static class Node2{
			Node2 left = null;
			Node2 right = null;
			Node2 parent = null;
			int data;
		}
		static class BinarySearchTree{
			Node2 root = null;
			int height = 0;
			int max = 0;
			int cnt = 1;
			ArrayList<Integer> parent = new ArrayList<>();
			HashMap<Integer, Integer> hm = new HashMap<>();
			public void insert(int x){
				Node2 n = new Node2();
				n.data = x;
				if(root==null){
					root = n;
				}
				else{
					Node2 temp = root,temp2 = null;
					while(temp!=null){
						temp2 = temp;
						if(x>temp.data) temp = temp.right;
						else temp = temp.left;
					}
					if(x>temp2.data) temp2.right = n;
					else temp2.left = n;
					n.parent = temp2;
					parent.add(temp2.data);
				}
			}
			public Node2 getSomething(int x, int y, Node2 n){
				if(n.data==x || n.data==y) return n;
				else if(n.data>x && n.data<y) return n;
				else if(n.data<x && n.data<y) return getSomething(x,y,n.right);
				else return getSomething(x,y,n.left);
			}
			public Node2 search(int x,Node2 n){
				if(x==n.data){
					max = Math.max(max, n.data);
					return n;
				}
				if(x>n.data){
					max = Math.max(max, n.data);
					return search(x,n.right);
				}
				else{
					max = Math.max(max, n.data);
					return search(x,n.left);
				}
			}
			public int getHeight(Node2 n){
				if(n==null) return 0;
				height = 1+ Math.max(getHeight(n.left), getHeight(n.right));
				return height;
			}
		}
		static long findDiff(long[] arr, long[] brr, int m){
			int i = 0, j = 0;
			long fa = 1000000000000L;
			while(i<m && j<m){
				long x = arr[i]-brr[j];
				if(x>=0){
					if(x<fa) fa = x;
					j++;
				}
				else{
					if((-x)<fa) fa = -x;
					i++;
				}
			}
			return fa;
		}
		public static long max(long x, long y, long z){
			if(x>=y && x>=z) return x;
			if(y>=x && y>=z) return y;
			return z;
		}
		
		static long modInverse(long a, long mOD2){
		            return  power(a, mOD2-2, mOD2);
		}
		static long power(long x, long y, long m)
		{
		    if (y == 0)
		        return 1;
		    long p = power(x, y/2, m) % m;
		    p = (p * p) % m;
		 
		    return (y%2 == 0)? p : (x * p) % m;
		}
		static long power2(long x,BigInteger y,long m){
			long ans = 1;
			BigInteger two = new BigInteger("2");
			while(y.compareTo(BigInteger.ZERO)>0){
				if(y.getLowestSetBit()==y.bitCount()){
					x = (x*x)%MOD;
					y = y.divide(two);
				}
				else{
					ans = (ans*x)%MOD;
					y = y.subtract(BigInteger.ONE);
				}
			}
			return ans;
		}
		static BigInteger power2(BigInteger x, BigInteger y, BigInteger m){
			BigInteger ans = new BigInteger("1");
			BigInteger one  = new BigInteger("1");
			BigInteger two  = new BigInteger("2");
			BigInteger zero  = new BigInteger("0");
			while(y.compareTo(zero)>0){
				//System.out.println(y);
				if(y.mod(two).equals(one)){
					ans = ans.multiply(x).mod(m);
					y = y.subtract(one);
				}
				else{
					x = x.multiply(x).mod(m);
					y = y.divide(two);
				}
			}
			return ans;
		}
		static BigInteger power(BigInteger x, BigInteger y, BigInteger m)
		{
		    if (y.equals(0))
		        return (new BigInteger("1"));
		    BigInteger p = power(x, y.divide(new BigInteger("2")), m).mod(m);
		    p = (p.multiply(p)).mod(m);
		 
		    return (y.mod(new BigInteger("2")).equals(0))? p : (p.multiply(x)).mod(m);
		}
		static long d,x,y;
		public static void extendedEuclidian(long a, long b){
			if(b == 0) {
		        d = a;
		        x = 1;
		        y = 0;
		    }
		    else {
		        extendedEuclidian(b, a%b);
		        int temp = (int) x;
		        x = y;
		        y = temp - (a/b)*y;
		    }
		}
		
		public static long gcd(long n, long m){
			if(m!=0) return gcd(m,n%m);
			else return n;
		}
		
		static BufferedReader reader;
	    static StringTokenizer tokenizer;
	    static PrintWriter writer;
	
	
	   
	    static class FasterScanner {

			private final InputStream stream;
			private final byte[] buf = new byte[8192];
			private int curChar, snumChars;
			private SpaceCharFilter filter;

			public FasterScanner(InputStream stream) {
				this.stream = stream;
			}

			public int snext() {
				if (snumChars == -1)
					throw new InputMismatchException();
				if (curChar >= snumChars) {
					curChar = 0;
					try {
						snumChars = stream.read(buf);
					} catch (IOException e) {
						throw new InputMismatchException();
					}
					if (snumChars <= 0)
						return -1;
				}
				return buf[curChar++];
			}

			public int nextInt() {
				int c = snext();
				while (isSpaceChar(c)) {
					c = snext();
				}
				int sgn = 1;
				if (c == '-') {
					sgn = -1;
					c = snext();
				}
				int res = 0;
				do {
					if (c < '0' || c > '9')
						throw new InputMismatchException();
					res *= 10;
					res += c - '0';
					c = snext();
				} while (!isSpaceChar(c));
				return res * sgn;
			}

			public long nextLong() {
				int c = snext();
				while (isSpaceChar(c)) {
					c = snext();
				}
				int sgn = 1;
				if (c == '-') {
					sgn = -1;
					c = snext();
				}
				long res = 0;
				do {
					if (c < '0' || c > '9')
						throw new InputMismatchException();
					res *= 10;
					res += c - '0';
					c = snext();
				} while (!isSpaceChar(c));
				return res * sgn;
			}

			public int[] nextIntArray(int n) {
				int a[] = new int[n];
				for (int i = 0; i < n; i++) {
					a[i] = nextInt();
				}
				return a;
			}

			public long[] nextLongArray(int n) {
				long a[] = new long[n];
				for (int i = 0; i < n; i++) {
					a[i] = nextInt();
				}
				return a;
			}
	                
			public String readString() {
				int c = snext();
				while (isSpaceChar(c)) {
					c = snext();
				}
				StringBuilder res = new StringBuilder();
				do {
					res.appendCodePoint(c);
					c = snext();
				} while (!isSpaceChar(c));
				return res.toString();
			}

			public String nextLine() {
				int c = snext();
				while (isSpaceChar(c))
					c = snext();
				StringBuilder res = new StringBuilder();
				do {
					res.appendCodePoint(c);
					c = snext();
				} while (!isEndOfLine(c));
				return res.toString();
			}

			public boolean isSpaceChar(int c) {
				if (filter != null)
					return filter.isSpaceChar(c);
				return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
			}

			private boolean isEndOfLine(int c) {
				return c == '\n' || c == '\r' || c == -1;
			}

			public interface SpaceCharFilter {
				public boolean isSpaceChar(int ch);
			}
		}
	}  