using System;
using System.Linq;
using System.Collections.Generic;
using Debug = System.Diagnostics.Debug;
using SB = System.Text.StringBuilder;
using System.Numerics;
using static System.Math;
namespace Program {
    public class Solver {
        Random rnd = new Random(0);
        public void Solve() {
            var n = ri;
            var A = Enumerate(n, x => rl);
            var G = Enumerate(n, x => new List<int>());
            for (int i = 0; i < n - 1; i++) {
                var u = ri - 1;
                var v = ri - 1;
                G[u].Add(v);
                G[v].Add(u);
            }
            // all plus
            var dp = Enumerate(n, x => new long[0]);

            // other case
            var ep = Enumerate(n, x => new long[0]);
            const long MAX = 5050 * 1000000000L;

            Action<int, int> dfs = null;
            dfs = (prev, cur) =>
            {
                var d = new long[1];
                var e = new long[1];
                d[0] = INF;
                e[0] = INF;
                if (A[cur] > 0) d[0] = A[cur];
                else e[0] = A[cur];

                foreach (var t in G[cur]) {
                    if (t == prev) continue;
                    dfs(cur, t);
                    var dd = dp[t];
                    var ee = ep[t];

                    var nd = new long[d.Length + dd.Length];
                    var ne = new long[e.Length + ee.Length];
                    for (int i = 0; i < nd.Length; i++)
                        nd[i] = ne[i] = INF;

                    for (int i = 0; i < d.Length; i++)
                        for (int j = 0; j < dd.Length; j++) {
                            nd[i + j] = Min(nd[i + j], d[i] + dd[j]);

                            ne[i + j] = Min(ne[i + j], e[i] + ee[j]);
                            ne[i + j] = Min(ne[i + j], d[i] + ee[j]);
                            ne[i + j] = Min(ne[i + j], e[i] + dd[j]);
                            if (dd[j] < MAX || ee[j] < 0) {
                                nd[i + j + 1] = Min(nd[i + j + 1], d[i]);
                                ne[i + j + 1] = Min(ne[i + j + 1], e[i]);
                            }
                        }

                    d = nd;
                    e = ne;
                }
                dp[cur] = d;
                ep[cur] = e;
            };
            dfs(-1, 0);
            for (int i = 0; i < n; i++) {
                Debug.WriteLine(i);
                Debug.WriteLine(dp[i].AsJoinedString());
                Debug.WriteLine(ep[i].AsJoinedString());
            }
            for (int i = 0; i < dp[0].Length; i++) {
                if (dp[0][i] < MAX || (ep[0][i] < 0)) {
                    Console.WriteLine(i);
                    return;
                }
            }
        }

        const long INF = 1L << 60;
        int ri { get { return sc.Integer(); } }
        long rl { get { return sc.Long(); } }
        double rd { get { return sc.Double(); } }
        string rs { get { return sc.Scan(); } }
        public IO.StreamScanner sc = new IO.StreamScanner(Console.OpenStandardInput());

        static T[] Enumerate<T>(int n, Func<int, T> f) {
            var a = new T[n];
            for (int i = 0; i < n; ++i) a[i] = f(i);
            return a;
        }
        static public void Swap<T>(ref T a, ref T b) { var tmp = a; a = b; b = tmp; }
    }
}

#region main
static class Ex {
    static public string AsString(this IEnumerable<char> ie) { return new string(ie.ToArray()); }
    static public string AsJoinedString<T>(this IEnumerable<T> ie, string st = " ") {
        return string.Join(st, ie);
    }
    static public void Main() {
        Console.SetOut(new Program.IO.Printer(Console.OpenStandardOutput()) { AutoFlush = false });
        var solver = new Program.Solver();
        solver.Solve();
        Console.Out.Flush();
    }
}
#endregion
#region Ex
namespace Program.IO {
    using System.IO;
    using System.Text;
    using System.Globalization;

    public class Printer: StreamWriter {
        public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
        public Printer(Stream stream) : base(stream, new UTF8Encoding(false, true)) { }
    }

    public class StreamScanner {
        public StreamScanner(Stream stream) { str = stream; }

        public readonly Stream str;
        private readonly byte[] buf = new byte[1024];
        private int len, ptr;
        public bool isEof = false;
        public bool IsEndOfStream { get { return isEof; } }

        private byte read() {
            if (isEof) return 0;
            if (ptr >= len) {
                ptr = 0;
                if ((len = str.Read(buf, 0, 1024)) <= 0) {
                    isEof = true;
                    return 0;
                }
            }
            return buf[ptr++];
        }

        public char Char() {
            byte b = 0;
            do b = read(); while ((b < 33 || 126 < b) && !isEof);
            return (char)b;
        }
        public string Scan() {
            var sb = new StringBuilder();
            for (var b = Char(); b >= 33 && b <= 126; b = (char)read()) sb.Append(b);
            return sb.ToString();
        }
        public string ScanLine() {
            var sb = new StringBuilder();
            for (var b = Char(); b != '\n' && b != 0; b = (char)read()) if (b != '\r') sb.Append(b);
            return sb.ToString();
        }
        public long Long() { return isEof ? long.MinValue : long.Parse(Scan()); }
        public int Integer() { return isEof ? int.MinValue : int.Parse(Scan()); }
        public double Double() { return isEof ? double.NaN : double.Parse(Scan(), CultureInfo.InvariantCulture); }
    }
}

#endregion

Measured memory_bytes: 201368