#include <bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i, n) for (long long i = 0, max_i = (n); i < max_i; i++)
#define REPI(i, a, b) for (long long i = (a), max_i = (b); i < max_i; i++)
#define ALL(obj) begin(obj), end(obj)
#define RALL(obj) rbegin(obj), rend(obj)
#define fi first
#define se second
using ii = pair<int, int>;
vector<ii> dirs = {
  {1, 0}, {0, 1}, {-1, 0}, {0, -1},  // 4方向
  {1, 1}, {-1, 1}, {-1, -1}, {1, -1},  // 斜め
  {0, 0},  // 自身
};
template <class T> inline bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } return false; }
template <class T> inline bool chmin(T& a, const T& b) { if (a > b) { a = b; return true; } return false; }
template <class T, class S> vector<T> make_vec(size_t n, S x) { return vector<T>(n, x); }
template <class T, class... Ts> auto make_vec(size_t n, Ts... ts) { return vector<decltype(make_vec<T>(ts...))>(n, make_vec<T>(ts...)); }

// debug
template <class T> ostream& operator<<(ostream& s, vector<T>& d) { REP (i, d.size()) s << d[i] << (i == d.size() - 1 ? "" : " "); return s; }
template <class T> ostream& operator<<(ostream& s, vector<vector<T>>& d) { REP (i, d.size()) s << d[i] << (i == d.size() - 1 ? "" : "\n"); return s; }
template <class T, class S> ostream& operator<<(ostream& s, pair<T, S>& p) { s << "{" << p.first << ", " << p.second << "}"; return s; }
template <class T, class S> ostream& operator<<(ostream& s, map<T, S> m) { for (auto it = m.begin(); it != m.end(); it++) { s << *it << (next(it) == m.end() ? "" : "\n"); } return s; }
#ifdef _MY_DEBUG
  #define dump(...) cerr << "/* " << #__VA_ARGS__ << " :[" << __LINE__ << ":" << __FUNCTION__ << "]" << endl, dump_func(__VA_ARGS__), cerr << "*/\n\n";
#else
  #define dump(...)
  #define endl "\n"
#endif
void dump_func() { cerr << endl; }
template <class Head, class... Tail> void dump_func(Head&& h, Tail&&... t) { cerr << h << (sizeof...(Tail) == 0 ? "" : ", "), dump_func(forward<Tail>(t)...); }

struct Fast { Fast() { cin.tie(0); ios::sync_with_stdio(false); } } fast;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
constexpr int MOD = 1000000007;
// *************** TEMPLATE END *************** 

signed main() {
  int n; cin >> n;
  string s; cin >> s;
  vector<vector<int>> g(n);
  REP (i, n - 1) {
    int u, v; cin >> u >> v;
    u--;
    v--;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  vector<int> upmin(n), upmax(n), num(n);
  function<void(int, int)> dfs = [&](int v, int p) {
    num[v] = s[v] == '1';
    int sum = 0, ma = 0, ima = -1;
    for (int s : g[v]) if (s != p) {
      dfs(s, v);
      num[v] += num[s];
      upmax[v] += upmax[s] + num[s];
      sum += upmax[s] + num[s];
      if (ma <= upmin[s] + num[s]) {
        ma = upmin[s] + num[s];
        ima = s;
      }
    }
    if (ima == -1) return;
    sum += ma - (upmax[ima] + num[ima]);
    int res = sum - ma;
    upmin[v] = max(sum % 2, ma - res);
  };
  const int INF = 1e10;
  int ans = INF;
  REP (i, n) {
    upmin = vector<int>(n);
    upmax = vector<int>(n);
    num = vector<int>(n);
    dfs(i, - 1);
    if (upmin[i] == 0) {
      chmin(ans, upmax[i] / 2);
    }
  }
  cout << (ans == INF ? -1 : ans) << endl;
}


Measured memory_bytes: 640