import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.complex, std.container, std.math, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum INF = 10L^^18;

int N;
string S;
int[] A, B;

int[][] G;
long total;
long[] cnt, sum;

void dfs(int u, int p) {
  cnt[u] = S[u] - '0';
  long sumY, maxY;
  foreach (v; G[u]) {
    if (v != p) {
      dfs(v, u);
      cnt[u] += cnt[v];
      const y = cnt[v] + sum[v];
      sumY += y;
      chmax(maxY, y);
    }
  }
  if (maxY <= sumY - maxY) {
    total += sumY / 2;
    sum[u] = sumY % 2;
  } else {
    total += sumY - maxY;
    sum[u] = maxY - (sumY - maxY);
  }
}

void main() {
  try {
    for (; ; ) {
      N = readInt();
      S = readToken();
      A = new int[N - 1];
      B = new int[N - 1];
      foreach (i; 0 .. N - 1) {
        A[i] = readInt() - 1;
        B[i] = readInt() - 1;
      }
      
      G = new int[][N];
      foreach (i; 0 .. N - 1) {
        G[A[i]] ~= B[i];
        G[B[i]] ~= A[i];
      }
      
      long ans = INF;
      foreach (r; 0 .. N) {
        debug {
          writeln("r = ", r);
        }
        total = 0;
        cnt = new long[N];
        sum = new long[N];
        dfs(r, -1);
        debug {
          writeln("  cnt = ", cnt);
          writeln("  sum = ", sum);
        }
        if (sum[r] == 0) {
          chmin(ans, total);
        }
      }
      writeln((ans >= INF) ? -1 : ans);
    }
  } catch (EOFException e) {
  }
}
