#include <bits/stdc++.h>
#include <sys/time.h>
using namespace std;

#define rep(i,n) for(long long i = 0; i < (long long)(n); i++)
#define repi(i,a,b) for(long long i = (long long)(a); i < (long long)(b); i++)
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second
#define mt make_tuple
#define mp make_pair
template<class T1, class T2> bool chmin(T1 &a, T2 b) { return b < a && (a = b, true); }
template<class T1, class T2> bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

using ll = long long; using vll = vector<ll>; using vvll = vector<vll>; using P = pair<ll, ll>;
ll ugauss(ll a, ll b) { if (!a) return 0; if (a>0^b>0) return a/b; else return (a+(a>0?-1:1))/b+1; }
ll lgauss(ll a, ll b) { if (!a) return 0; if (a>0^b>0) return (a+(a>0?-1:1))/b-1; else return a/b; }
template <typename T, typename U> ostream &operator<<(ostream &o, const pair<T, U> &v) {  o << "(" << v.first << ", " << v.second << ")"; return o; }
template<size_t...> struct seq{}; template<size_t N, size_t... Is> struct gen_seq : gen_seq<N-1, N-1, Is...>{}; template<size_t... Is> struct gen_seq<0, Is...> : seq<Is...>{};
template<class Ch, class Tr, class Tuple, size_t... Is>
void print_tuple(basic_ostream<Ch,Tr>& os, Tuple const& t, seq<Is...>){ using s = int[]; (void)s{0, (void(os << (Is == 0? "" : ", ") << get<Is>(t)), 0)...}; }
template<class Ch, class Tr, class... Args> 
auto operator<<(basic_ostream<Ch, Tr>& os, tuple<Args...> const& t) -> basic_ostream<Ch, Tr>& { os << "("; print_tuple(os, t, gen_seq<sizeof...(Args)>()); return os << ")"; }
ostream &operator<<(ostream &o, const vvll &v) { rep(i, v.size()) { rep(j, v[i].size()) o << v[i][j] << " "; o << endl; } return o; }
template <typename T> ostream &operator<<(ostream &o, const vector<T> &v) { o << '['; rep(i, v.size()) o << v[i] << (i != v.size()-1 ? ", " : ""); o << "]";  return o; }
template <typename T> ostream &operator<<(ostream &o, const deque<T> &v) { o << '['; rep(i, v.size()) o << v[i] << (i != v.size()-1 ? ", " : ""); o << "]";  return o; }
template <typename T>  ostream &operator<<(ostream &o, const set<T> &m) { o << '['; for (auto it = m.begin(); it != m.end(); it++) o << *it << (next(it) != m.end() ? ", " : ""); o << "]";  return o; }
template <typename T>  ostream &operator<<(ostream &o, const unordered_set<T> &m) { o << '['; for (auto it = m.begin(); it != m.end(); it++) o << *it << (next(it) != m.end() ? ", " : ""); o << "]";  return o; }
template <typename T, typename U>  ostream &operator<<(ostream &o, const map<T, U> &m) { o << '['; for (auto it = m.begin(); it != m.end(); it++) o << *it << (next(it) != m.end() ? ", " : ""); o << "]";  return o; }
template <typename T, typename U, typename V>  ostream &operator<<(ostream &o, const unordered_map<T, U, V> &m) { o << '['; for (auto it = m.begin(); it != m.end(); it++) o << *it; o << "]";  return o; }
vector<int> range(const int x, const int y) { vector<int> v(y - x + 1); iota(v.begin(), v.end(), x); return v; }
template <typename T> istream& operator>>(istream& i, vector<T>& o) { rep(j, o.size()) i >> o[j]; return i;}
template <typename T, typename S, typename U> ostream &operator<<(ostream &o, const priority_queue<T, S, U> &v) { auto tmp = v; while (tmp.size()) { auto x = tmp.top(); tmp.pop(); o << x << " ";} return o; }
template <typename T> ostream &operator<<(ostream &o, const queue<T> &v) { auto tmp = v; while (tmp.size()) { auto x = tmp.front(); tmp.pop(); o << x << " ";} return o; }
template <typename T> ostream &operator<<(ostream &o, const stack<T> &v) { auto tmp = v; while (tmp.size()) { auto x = tmp.top(); tmp.pop(); o << x << " ";} return o; }
template <typename T> unordered_map<T, ll> counter(vector<T> vec){unordered_map<T, ll> ret; for (auto&& x : vec) ret[x]++; return ret;};
void vizGraph(vvll& g, int mode = 0, string filename = "out.png") { ofstream ofs("./out.dot"); ofs << "digraph graph_name {" << endl; set<P> memo; rep(i, g.size())  rep(j, g[i].size()) { if (mode && (memo.count(P(i, g[i][j])) || memo.count(P(g[i][j], i)))) continue; memo.insert(P(i, g[i][j])); ofs << "    " << i << " -> " << g[i][j] << (mode ? " [arrowhead = none]" : "")<< endl;  } ofs << "}" << endl; ofs.close(); system(((string)"dot -T png out.dot >" + filename).c_str()); }
struct timeval start; double sec() { struct timeval tv; gettimeofday(&tv, NULL); return (tv.tv_sec - start.tv_sec) + (tv.tv_usec - start.tv_usec) * 1e-6; }
size_t random_seed; struct init_{init_(){ ios::sync_with_stdio(false); cin.tie(0); gettimeofday(&start, NULL); struct timeval myTime; struct tm *time_st; gettimeofday(&myTime, NULL); time_st = localtime(&myTime.tv_sec); srand(myTime.tv_usec); random_seed = RAND_MAX / 2 + rand() / 2; }} init__;
#define ldout fixed << setprecision(40) 

#define EPS (double)1e-14
#define INF (ll)1e18
#define mo  (ll)(1e9+7)
ll n;
vll a;
vll u, v;
vvll g;
ll dp[5010][5010][2] = {};
ll tmp[5010][2];
ll tsize[5010];
ll dfs(ll v, ll p) {
    ll ret = 1;
    vector<P> c;
    for (auto x : g[v]) if (x != p) {
        ll tmp = dfs(x, v);
        c.pb(P(x, tmp));
        ret += tmp;
    }
    dp[v][0][0] = a[v] > 0;
    dp[v][0][1] = a[v];

    ll lsize = 1;
    for (auto is : c) {
//        cout << is << "#child" << endl;
        ll ci = is.fi;
        ll rsize = is.se;
        rep(i, lsize+rsize) {
            tmp[i][0] = 0;
            tmp[i][1] = INF;
        }
        rep(i, lsize) rep(j, rsize) {
            if (dp[v][i][0] == 1 && dp[ci][j][0] == 1) {
                tmp[i+j][0] = 1;
            }
            if (dp[v][i][0] == 1 && (dp[ci][j][1] < 0 || dp[ci][j][0])) {
                tmp[i+j+1][0] = 1;
            } 
            if (dp[v][i][1] != INF &&  dp[ci][j][1] != INF) {
                chmin(tmp[i+j+0][1], dp[v][i][1] + dp[ci][j][1]);
                if (dp[ci][j][1] < 0 || dp[ci][j][0]) {
                    chmin(tmp[i+j+1][1], dp[v][i][1]);
                }
            }
        }
        rep(i, lsize+rsize) rep(j, 2) {
            dp[v][i][j] = tmp[i][j];
//            cout << mt(v, i, j, dp[v][i][j]) << endl;
        }
        lsize += rsize;
    }
    return tsize[v] = ret;
}

int main(void) {
    cin >> n;
    a.resize(n);
    u.resize(n-1);
    v.resize(n-1);
    g.resize(n);
    cin >> a;
    rep(i, n-1) {
        cin >> u[i] >> v[i];
        u[i]--, v[i]--;
        g[u[i]].pb(v[i]);
        g[v[i]].pb(u[i]);
    }
    dfs(0, -1);

    rep(i, n) {
//        cout << i << "###########" << endl;
//        rep(j, tsize[i]) cout << dp[i][j][0] << " "; cout << endl;
//        rep(j, tsize[i]) cout << dp[i][j][1] << " "; cout << endl;
    }
    rep(i, n) {
        if (dp[0][i][0]) {
            cout << i << endl;
            return 0;
        } 
        if (dp[0][i][1] < 0) {
            cout << i << endl;
            return 0;
        } 
    }

    return 0;
}


Measured memory_bytes: 372732