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

using namespace std;

#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}


typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const ll MOD = 1000000007;
//const ll MOD = 998244353;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p<<endl;
int dx[4] = { -1,0, 1,0 }, dy[4] = { 0,1,0,-1 };
int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
//#define mp make_pair

int n;
string s;
vii G;

#define P pair<int, pii>

pii pre_dfs(int now, int par) {
  int sum = 0;
  int res = 0;
  if (s[now] == '1') sum++;
  rep (i, G[now].size()) {
    int ch = G[now][i];
    if (ch == par) {
      continue;
    }
    pii temp = pre_dfs(ch, now);
    sum += temp.first;
    res += temp.second;
  }
  if (par != -1) {
    return pii(sum, sum + res);
  }
  else {
    return pii(sum, res);
  }
}

P dfs(int now, int par) {
  int sum = 0;
  int ma = 0;
  vector<P> ch_p;
  if (s[now] == '1') sum++;
  rep (i, G[now].size()) {
    int ch = G[now][i];
    if (ch == par) {
      continue;
    }
    P res = dfs(ch, now);
    ch_p.push_back(res);
    sum += res.first;
    ma += res.second.second;
  }

  int mima = 0;
  rep (i, ch_p.size()) {
    int oma = ma - ch_p[i].second.second;
    int l = ch_p[i].second.first;
    mima = max(mima, l - oma);
  }
  if (par != -1) {
    return {sum, pii(mima + sum, ma + sum)};
  }
  else {
    return {sum, pii(mima, ma)};
  }
}


int main() {
  fio();
  cin >> n;
  cin >> s;
  G.resize(n);
  rep (i, n - 1) {
    int u, v;
    cin >> u >> v;
    u--; v--;
    G[u].push_back(v);
    G[v].push_back(u);
  }


  int ans = inf;
  rep (i, n) {
    pii temp = pre_dfs(i, -1);
    if (temp.second % 2 == 1) {
      continue;
    }
    P res = dfs(i, -1);
    //DEBUG(i);
    //DEBUG(res.second);
    if (res.second.first == 0) {
      ans = min(ans, temp.second / 2);
    }
  }
  if (ans == inf) {
    cout << -1 << endl;
  }
  else {
    cout << ans << endl;
  }
  
}


Measured memory_bytes: 640