//#define NDEBUG
#include <cstddef>
#include <cstdint>
#include <vector>

namespace n91 {

using i8 = std::int_fast8_t;
using i32 = std::int_fast32_t;
using i64 = std::int_fast64_t;
using u8 = std::uint_fast8_t;
using u32 = std::uint_fast32_t;
using u64 = std::uint_fast64_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

class rep {
  const usize f, l;

public:
  class itr {
    friend rep;
    usize i;
    constexpr itr(const usize x) noexcept : i(x) {}

  public:
    void operator++() noexcept { ++i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  constexpr rep(const usize first, const usize last) noexcept
      : f(first), l(last) {}
  constexpr itr begin() const noexcept { return itr(f); }
  constexpr itr end() const noexcept { return itr(l); }
};
class revrep {
  const usize f, l;

public:
  class itr {
    friend revrep;
    usize i;
    constexpr itr(usize x) noexcept : i(x) {}

  public:
    void operator++() noexcept { --i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  constexpr revrep(usize first, usize last) noexcept : f(--first), l(--last) {}
  constexpr itr begin() const noexcept { return itr(l); }
  constexpr itr end() const noexcept { return itr(f); }
};
template <class T> using vec_alias = std::vector<T>;
template <class T> auto md_vec(const usize n, const T &value) {
  return std::vector<T>(n, value);
}
template <class... Args> auto md_vec(const usize n, Args... args) {
  return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
}
template <class T> constexpr T difference(const T &a, const T &b) {
  return a < b ? b - a : a - b;
}

} // namespace n91

#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <utility>

namespace n91 {

void main_() {
  usize n;
  std::cin >> n;
  std::string s;
  std::cin >> s;
  auto g = md_vec(n, static_cast<usize>(0), static_cast<usize>(0));
  for (const auto i : rep(1, n)) {
    usize a, b;
    std::cin >> a >> b;
    --a;
    --b;
    g[a].emplace_back(b);
    g[b].emplace_back(a);
  }
  struct node {
    u64 count, min, max;
  };
  std::vector<node> dp(n);
  constexpr u64 INF = std::numeric_limits<u64>::max();
  const auto dfs = [&](const auto &dfs, const usize v, const usize p) -> void {
    dp[v].count = static_cast<u64>(0);
    dp[v].min = static_cast<u64>(0);
    dp[v].max = static_cast<u64>(0);
    if (s[v] == '1') {
      ++dp[v].count;
    }
    u64 max_max = static_cast<u64>(0), max_min = static_cast<u64>(0),
        other = static_cast<u64>(0);
    for (const auto e : g[v]) {
      if (e == p) {
        continue;
      }
      dfs(dfs, e, v);
      dp[v].count += dp[e].count;
      dp[v].max += dp[e].max + dp[e].count;
      if (dp[e].max + dp[e].count >= max_max) {
        other += max_max;
        max_max = dp[e].max + dp[e].count;
        max_min = dp[e].min + dp[e].count;
      } else {
        other += dp[e].max + dp[e].count;
      }
    }
    if (dp[v].max % static_cast<u64>(2) == static_cast<u64>(0)) {
      if (other >= max_min) {
        dp[v].min = static_cast<u64>(0);
      } else {
        dp[v].min = max_min - other;
      }
    } else {
      if (other >= max_min) {
        dp[v].min = static_cast<u64>(1);
      } else {
        dp[v].min = max_min - other;
      }
    }
  };
  u64 ans = INF;
  for (const auto i : rep(0, n)) {
    dfs(dfs, i, n);
    if (dp[i].min == static_cast<u64>(0)) {
      ans = std::min(ans, dp[i].max / static_cast<u64>(2));
    }
  }
  if (ans == INF) {
    std::cout << "-1";
  } else {
    std::cout << ans;
  }
  std::cout << std::endl;
}

} // namespace n91

int main() {
  n91::main_();
  return 0;
}


Measured memory_bytes: 512