import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, 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)); }


int N;
int[] A, B;
int[] C;

int[][] graph;

int ans;
int dfs(int u, int p) {
  int ret = C[u] ^ 1;
  foreach (v; graph[u]) if (p != v) {
    const res = dfs(v, u);
    if (res) {
      ++ans;
      ret ^= 1;
    }
  }
  return ret;
}

void main() {
  try {
    for (; ; ) {
      N = readInt;
      A = new int[N - 1];
      B = new int[N - 1];
      foreach (i; 0 .. N - 1) {
        A[i] = readInt - 1;
        B[i] = readInt - 1;
      }
      C = new int[N];
      foreach (u; 0 .. N) {
        C[u] = readInt;
      }
      
      graph = new int[][N];
      foreach (i; 0 .. N - 1) {
        graph[A[i]] ~= B[i];
        graph[B[i]] ~= A[i];
      }
      ans = 0;
      const res = dfs(0, -1);
      writeln(res ? -1 : ans);
    }
  } catch (EOFException e) {
  }
}