#include <iostream>
#include <fstream>
#include <cmath>  
#include <cstdlib>
#include <ctime>
#include <algorithm>
#include <numeric>
#include <functional>
#include <string> 
#include <vector>
#include <bitset>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <deque>

using namespace std;
using ll = long long;

#define REP(i,n) for(long long i = 0; i < (n); i++)
#define FOR(i, m, n) for(long long i = (m);i < (n); ++i)
#define ALL(obj) (obj).begin(),(obj).end()

template<class T> using V = vector<T>;
template<class T, class U> using P = pair<T, U>;

const ll MOD = (ll)1e9 + 7;
const ll MOD2 = 998244353;
const ll LLINF = (ll)1e18;
const ll INTINF = (ll)1e9;
const long double PI = 3.1415926535897932384626433;

template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
template <template <class tmp>  class T, class U> ostream &operator<<(ostream &o, const T<U> &obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr)o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max<T>(a,b);}
template <class T> void chmin(T& a, const T b){a=min<T>(a,b);}
void YN(bool flg) {cout << ((flg) ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << ((flg) ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << ((flg) ? "yes" : "no") << endl;}

int main() {
	int N; cin >> N;
	V<ll> A(N); REP(i,N) cin >> A[i];
	V<V<int>> edge(N);
	REP(i,N-1){
		int U,V; cin >> U >> V;
		U--,V--;
		edge[U].push_back(V);
		edge[V].push_back(U);		
	}
	V<int> depth(N,-1),scale(N,1),parent(N,-1);
	depth[0] = 0;
	queue<int> q; q.push(0);
	while(q.size()){
		int from = q.front();
		q.pop();
		for(auto to: edge[from]){
			if(depth[to]!=-1) continue;
			depth[to] = depth[from] + 1;
			parent[to] = from;
			q.push(to);
		}
	}
	V<P<int,int>> ord(N);
	REP(i,N) ord[i] = {-depth[i],i};
	sort(ALL(ord));
	
	V<V<ll>> dp1(N,V<ll>(N,LLINF)),dp2(N,V<ll>(N,LLINF));
	REP(i,N) (A[i]>0?dp1:dp2)[i][0] = A[i];

	for(int i = 0; i < N-1; ++i){
		int from = ord[i].second, to = parent[from];
		if(depth[to]>depth[from]) continue;
		
		V<ll> tmp1(N,LLINF),tmp2(N,LLINF);
		for(int j = 0; j < scale[to]; ++j){
			for(int k = 0; k < scale[from]; ++k){
				// merge
				chmin(tmp1[j+k],dp1[to][j]+dp1[from][k]);
				chmin(tmp2[j+k],dp1[to][j]+dp2[from][k]);
				chmin(tmp2[j+k],dp2[to][j]+dp1[from][k]);
				chmin(tmp2[j+k],dp2[to][j]+dp2[from][k]);

				// divide
				if(dp1[from][k]<LLINF || dp2[from][k] < 0)chmin(tmp1[j+k+1],dp1[to][j]);
				if(dp1[from][k]<LLINF || dp2[from][k] < 0)chmin(tmp2[j+k+1],dp2[to][j]);

			}
		}
		dp1[to] = tmp1;
		dp2[to] = tmp2;
		scale[to] += scale[from];
	}

	int ans = N-1;
	for(int j = 0; j < N; ++j){
		if(dp1[0][j]<LLINF) chmin(ans,j);
		if(dp2[0][j]<0) chmin(ans,j);
	}
	cout << ans << endl;


	return 0;
}


Measured memory_bytes: 391936