Problem:
Score : 700 points

Problem Statement

There is a tree with N vertices, numbered 1 through N.
The i-th of the N-1 edges connects vertices a_i and b_i.

Currently, there are A_i stones placed on vertex i.
Determine whether it is possible to remove all the stones from the vertices by repeatedly performing the following operation:

Select a pair of different leaves. Then, remove exactly one stone from every vertex on the path between those two vertices.
Here, a leaf is a vertex of the tree whose degree is 1, and the selected leaves themselves are also considered as vertices on the path connecting them.

Note that the operation cannot be performed if there is a vertex with no stone on the path.

Constraints

2 ≦ N ≦ 10^5

1 ≦ a_i,b_i ≦ N

0 ≦ A_i ≦ 10^9

The given graph is a tree.

Input

The input is given from Standard Input in the following format:

N
A_1 A_2 … A_N
a_1 b_1
:
a_{N-1} b_{N-1}

Output

If it is possible to remove all the stones from the vertices, print YES. Otherwise, print NO.

Sample Input 1

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

Sample Output 1

YES

All the stones can be removed, as follows:

Select vertices 4 and 5. Then, there is one stone remaining on each vertex except 4.

Select vertices 1 and 5. Then, there is no stone on any vertex.

Sample Input 2

3
1 2 1
1 2
2 3

Sample Output 2

NO

Sample Input 3

6
3 2 2 2 2 2
1 2
2 3
1 4
1 5
4 6

Sample Output 3

YES

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.io.BufferedWriter;
import java.util.InputMismatchException;
import java.util.ArrayList;
import java.util.List;
import java.util.NoSuchElementException;
import java.math.BigInteger;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.Writer;
import java.io.IOException;
import java.util.Arrays;
import java.io.InputStream;
import java.util.Random;
import java.io.OutputStreamWriter;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 * @author ogiekako
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		MyScanner in = new MyScanner(inputStream);
		MyPrintWriter out = new MyPrintWriter(outputStream);
		TaskC solver = new TaskC();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskC {
    public void solve(int testNumber, MyScanner in, MyPrintWriter out) {
        int N = in.nextInt();
        if (N < 10000) throw new RuntimeException();
        A = new long[N];
        for (int i = 0; i < N; i++) A[i] = in.nextInt();
        Graph G = new Graph(N);
        for (int i = 0; i < N - 1; i++) {
            int a = in.nextInt() - 1;
            int b = in.nextInt() - 1;
            G.add(a,b);
            G.add(b,a);
        }
        int ans = -1;
        for(int i=0;i<10;i++) {
            int r = new Random().nextInt(N);
            long res = dfs(G, r, -1);
            int tmp = res == (lastDeg <= 1 ? A[r] : 0) ? 1 : 0;
            if (ans == -1) ans = tmp;
            else if (ans != tmp){
                ans = -2;
                break;
            }
        }
        if (ans >= 0) {
            out.println(ans==1 ? "YES" : "NO");
        } else {
            throw new RuntimeException();
}
    }

    long[] A;
    int lastDeg;

    private long dfs(Graph g, int root, int prev) {
        long S = 0;
        long max = 0;
        int deg = 0;
        for(Edge e : g.edges(root)) {
            if (e.to() != prev) {
                deg++;
                long v = dfs(g, e.to(), root);
                if (v < 0) return -1;
                S += v;
                max = Math.max(max, v);
            }
        }
        if (A[root] < max) return -1;
        long res = deg == 0 ? A[root] : S - 2 * (S - A[root]);
        if (res < 0) return -1;
        lastDeg = deg;
        return res;
    }
}

class MyScanner {
    private final InputStream in;

    public MyScanner(InputStream in) {
        this.in = in;
    }

    private static final int BUFSIZE = 65536;
    int bufLen;
    int bufPtr;
    byte[] buf = new byte[BUFSIZE];

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

    public int nextInt() {
        int c = read();
        if (c == -1) throw new NoSuchElementException();
        while (c != '-' && (c < '0' || '9' < c)) {
            c = read();
            if (c == -1) throw new NoSuchElementException();
        }
        if (c == '-') return -nextInt();
        int res = 0;
        do {
            res *= 10;
            res += c - '0';
            c = read();
        } while ('0' <= c && c <= '9');
        return res;
    }

}

class MyPrintWriter {
    PrintWriter out;

    public MyPrintWriter(OutputStream outputStream) {
        out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
    }

    public void println(Object... os) {
        if (os.length == 0) {
            out.println();
            return;
        }
        for (int i = 0; i < os.length - 1; i++) {
            out.print(os[i]);
            out.print(' ');
        }
        out.println(os[os.length - 1]);
    }

    public void close() {
        out.close();
    }

}

class Graph {
    final int vertexCount;
    private EdgeList[] edges;

    public Graph(int vertexCount) {
        this.vertexCount = vertexCount;
        edges = new EdgeList[vertexCount];
        for (int i = 0; i < vertexCount; i++) edges[i] = new EdgeList();
    }

    public void add(Edge edge) {
        edges[edge.from()].add(edge);
        Edge rev = edge.reversed();
        if (rev != null) {
            edges[rev.from()].add(rev);
        }
    }

    public void add(int from, int to) {
        add(new SimpleEdge(from, to));
    }

    public List<Edge> edges(int vertex) {
        return edges[vertex];
    }

    public String toString() {
        StringBuilder es = new StringBuilder();
        for (int i = 0; i < vertexCount; i++) {
            if (i > 0) es.append('\n');
            es.append(edges(i));
        }
        return "Graph{" +
                "edges=" + es +
                '}';
    }

    private static class EdgeList extends ArrayList<Edge> {
        EdgeList() {
            super(0);
        }
    }
}

interface Edge {
    int from();

    int to();

    Edge reversed();
}

class SimpleEdge implements Edge {
    final int from;
    final int to;

    public SimpleEdge(int from, int to) {
        this.from = from;
        this.to = to;
    }

    public String toString() {
        return from() + " -> " + to();
    }

    public int from() {
        return from;
    }

    public int to() {
        return to;
    }

    public Edge reversed() {
        return null;
    }

}