#include <bits/stdc++.h>
#include <random>

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;

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

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

const double PI = 2 * acos(0.0);
#define rand shittttty_shit
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng_64(chrono::steady_clock::now().time_since_epoch().count());

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
    return 0;
}

const int N = 2019;
bool have[N];
vi g[N];
int n;

triple(int) dfs(int v, int par) {
    triple(int) ret = make_tuple(0, 0, have[v]);
    for (int to : g[v]) if (to != par) {
        triple(int) cur = dfs(to, v);
        int f, s, t;
        tie(f, s, t) = cur;
        f += t;
        s += t;
        int ff, ss, tt;
        tie(ff, ss, tt) = ret;
        int a, b, c;
        b = ss + s;
        c = tt + t;
        if (ff > f) {
            swap(ff, f);
            swap(ss, s);
        }
        if (f > ss) {
            a = f - ss;
        } else {
            a = (f ^ ff) & 1;
        }
        ret = make_tuple(a, b, c);
    }
    return ret;
}

pii count_dfs(int v, int par) {
    pii ret = {0, have[v]};
    for (int to : g[v]) if (to != par) {
        pii cur = count_dfs(to, v);
        ret.first += cur.first + cur.second;
        ret.second += cur.second;
    }
    return ret;
}

void smain() {

    cin >> n;
    string s;
    cin >> s;
    fori (i, n) {
        have[i + 1] = s[i] == '1';
    }
    fori (i, n - 1) {
        int u, v;
        cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    int ans = -1;
    for (int v = 1; v <= n; ++v) {
        int f, s, t;
        tie(f, s, t) = dfs(v, -1);
        if (f == 0) {
            int cnt = count_dfs(v, -1).first;
            if (ans == -1) ans = cnt;
            else inmin(ans, cnt);
        }
    }
    if (ans != -1) ans >>= 1;
    cout << ans;

}


Measured memory_bytes: 512