#include <bits/stdc++.h>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef long double ld;
//#define int ll
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector< vi > vvi;
typedef vector< vvi > vvvi;
typedef vector<short> vs;
typedef vector<vs> vvs;
typedef vector<vvs> vvvs;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
typedef vector<ld> vld;
typedef vector<vld> vvld;
typedef vector<vvld> vvvld;
typedef vector<string> vst;
typedef vector<vst> vvst;
typedef pair<ld, ld> pld;
typedef complex<double> base;
typedef complex<int> point;

#define inmin(a, b) a = min(a, (b))
#define inmax(a, b) a = max(a, (b))
#define ALL(a) a.begin(),a.end()
#define RALL(a) a.rbegin(),a.rend()
#define sqr(x) ((x) * (x))
#define fori(i, n) for(int i = 0; i < int(n); ++i)
#define SZ(a) ((int)((a).size()))
#define triple(T) tuple<T, T, T>
#define quad(T) tuple<T, T, T, T>

#ifdef MAX_HOME
#define cerr cout
#else
#define cerr if (false) cerr
#endif

#define watch(x) cerr << (#x) << " = " << (x)    << endl;

const double PI = 2 * acos(0.0);

const string DIGITS = "0123456789";
const string ALPH = "abcdefghijklmnopqrstuvwxyz";

template <class T0, class T1>
inline ostream & operator << (ostream &out, pair<T0, T1> &a) {
    return out << "{" << a.first << ", " << a.second << "}";
}

template <class T0, class T1, class T2>
inline ostream & operator << (ostream &out, tuple<T0, T1, T2> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << "}";
}

template <class T0, class T1, class T2, class T3>
inline ostream & operator << (ostream &out, tuple<T0, T1, T2, T3> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << ", " <<  get<3>(a) << "}";
}

template<class T>
inline ostream & operator << (ostream &out, vector<T> &a) {
    out << "[";
    fori (i, a.size())
        out << a[i] << vector<string>{", ", "]  "}[i + 1 == a.size()];
    return out;
}



void smain();

signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);

#ifdef MAX_HOME
    freopen("input.txt", "r", stdin);
    clock_t start = clock();
#endif
    cout << setprecision(12) << fixed;
    smain();
#ifdef MAX_HOME
    cout << "\n\n\n\nTOTAL EXECUTION TIME: " << float( clock () - start ) /  CLOCKS_PER_SEC << endl;
#endif
}

const int N = 5009;

vi g[N], _g[N];
ll a[N];
int n;

void hang(int v, int par) {
    for (int to : _g[v]) if (to != par) {
        g[v].push_back(to);
        hang(to, v);
    }
}

vvvl dp(N, vvl(2));
const ll oo = 1e18;

vvl m_merge(vvl dp, vvl sdp) {
    int sz = max(dp[0].size(), dp[1].size()) + max(sdp[0].size(), sdp[1].size()) + 1;
    inmin(sz, n);
    vvl ret(2, vl(sz, oo));
    fori (z, 2) {
        fori (r, dp[z].size()) if (dp[z][r] < oo) {
            fori (k, 2) {
                fori (t, sdp[k].size()) if (sdp[k][t] < oo) {
                    int nz = z || k;
                    int ns = r + t;
                    if (ns < sz) inmin(ret[nz][ns], dp[z][r] + sdp[k][t]);
                    if ((k == 0 || sdp[k][t] < 0) && r + 1 + t < sz) inmin(ret[z][r + t + 1], dp[z][r]);
                }
            }
        }
    }
    return ret;
}

void dfs(int v) {
    if (g[v].empty()) {
        dp[v][1] = {a[v]};
        if (a[v] > 0) dp[v][0] = {a[v]};
    } else {
        dp[v][1] = {a[v]};
        if (a[v] > 0) dp[v][0] = {a[v]};

        for (int to : g[v]) {
            dfs(to);
            dp[v] = m_merge(dp[v], dp[to]);
//            watch(to);
        }
    }
}

void smain() {

    cin >> n;
    fori (i, n) cin >> a[i + 1];
    fori (i, n - 1) {
        int u, v;
        cin >> u >> v;
        _g[u].push_back(v);
        _g[v].push_back(u);
    }
    hang(1, -1);

    dfs(1);
    int ans = n - 1;
    fori (z, 2) {
        fori (r, dp[1][z].size()) if (dp[1][z][r] < oo) {
            if (!z || dp[1][z][r] < 0) inmin(ans, r);
        }
    }
    cout << ans;
}

Measured memory_bytes: 302848