import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.util.ArrayList;
import java.io.UncheckedIOException;
import java.util.List;
import java.io.Closeable;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.InputStream;

/**
 * Built using CHelper plug-in Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) throws Exception {
        Thread thread = new Thread(null, new TaskAdapter(), "", 1 << 27);
        thread.start();
        thread.join();
    }

    static class TaskAdapter implements Runnable {
        @Override
        public void run() {
            InputStream inputStream = System.in;
            OutputStream outputStream = System.out;
            FastInput in = new FastInput(inputStream);
            FastOutput out = new FastOutput(outputStream);
            TaskE solver = new TaskE();
            solver.solve(1, in, out);
            out.close();
        }
    }
    static class TaskE {
        public void solve(int testNumber, FastInput in, FastOutput out) {
            int n = in.readInt();
            Node[] nodes = new Node[n + 1];
            for (int i = 1; i <= n; i++) {
                nodes[i] = new Node();
            }
            for (int i = 1; i <= n; i++) {
                nodes[i].pieces = in.readChar() - '0';
            }
            for (int i = 1; i < n; i++) {
                Node a = nodes[in.readInt()];
                Node b = nodes[in.readInt()];
                a.next.add(b);
                b.next.add(a);
            }


            int inf = (int) 1e9;
            int ans = inf;
            for (int i = 1; i <= n; i++) {
                for (int j = 1; j <= n; j++) {
                    nodes[j].combine = nodes[j].oneCnt = nodes[j].sumOfHeights = 0;
                }
                dfs(nodes[i], null);
                if (nodes[i].combine * 2 != nodes[i].sumOfHeights) {
                    continue;
                }
                ans = Math.min(ans, nodes[i].combine);
            }

            out.println(ans == inf ? -1 : ans);
        }

        public void dfs(Node root, Node father) {
            int combineHappen = 0;
            int singleHeight = 0;

            for (Node node : root.next) {
                if (node == father) {
                    continue;
                }
                dfs(node, root);
                node.sumOfHeights += node.oneCnt;
                root.oneCnt += node.oneCnt;
                root.sumOfHeights += node.sumOfHeights;

                int h = node.sumOfHeights - node.combine * 2;
                if (h <= singleHeight) {
                    int extract = Math.min(node.combine, (singleHeight - h) / 2);
                    node.combine -= extract;
                    h += extract * 2;
                    singleHeight -= h;
                    combineHappen += h;
                    combineHappen += node.combine;
                } else {
                    int extract = Math.min((h - singleHeight) / 2, combineHappen);
                    combineHappen -= extract;
                    singleHeight += extract * 2;
                    combineHappen += singleHeight;
                    h -= singleHeight;
                    singleHeight = h;
                    combineHappen += node.combine;
                }
            }

            if (root.pieces == 1) {
                root.oneCnt++;
            }
            root.combine = combineHappen;
        }

    }
    static class FastOutput implements AutoCloseable, Closeable {
        private StringBuilder cache = new StringBuilder(10 << 20);
        private final Writer os;

        public FastOutput(Writer os) {
            this.os = os;
        }

        public FastOutput(OutputStream os) {
            this(new OutputStreamWriter(os));
        }

        public FastOutput println(int c) {
            cache.append(c).append('\n');
            return this;
        }

        public FastOutput flush() {
            try {
                os.append(cache);
                os.flush();
                cache.setLength(0);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
            return this;
        }

        public void close() {
            flush();
            try {
                os.close();
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        public String toString() {
            return cache.toString();
        }

    }
    static class FastInput {
        private final InputStream is;
        private byte[] buf = new byte[1 << 13];
        private int bufLen;
        private int bufOffset;
        private int next;

        public FastInput(InputStream is) {
            this.is = is;
        }

        private int read() {
            while (bufLen == bufOffset) {
                bufOffset = 0;
                try {
                    bufLen = is.read(buf);
                } catch (IOException e) {
                    bufLen = -1;
                }
                if (bufLen == -1) {
                    return -1;
                }
            }
            return buf[bufOffset++];
        }

        public void skipBlank() {
            while (next >= 0 && next <= 32) {
                next = read();
            }
        }

        public int readInt() {
            int sign = 1;

            skipBlank();
            if (next == '+' || next == '-') {
                sign = next == '+' ? 1 : -1;
                next = read();
            }

            int val = 0;
            if (sign == 1) {
                while (next >= '0' && next <= '9') {
                    val = val * 10 + next - '0';
                    next = read();
                }
            } else {
                while (next >= '0' && next <= '9') {
                    val = val * 10 - next + '0';
                    next = read();
                }
            }

            return val;
        }

        public char readChar() {
            skipBlank();
            char c = (char) next;
            next = read();
            return c;
        }

    }
    static class Node {
        List<Node> next = new ArrayList<>();
        int pieces;
        int combine;
        int sumOfHeights;
        int oneCnt;

    }
}



Measured memory_bytes: 84628