/* -*- coding: utf-8 -*-
 *
 * 2205.cc:  No.2205 Lights Out on Christmas Tree - yukicoder
 */

#include<cstdio>
#include<vector>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_N = 200000;

/* typedef */

typedef vector<int> vi;

/* global variables */

vi nbrs[MAX_N];
int cs[MAX_N], ps[MAX_N], ss[MAX_N], cis[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);

  for (int i = 1; i < n; i++) {
    int u, v;
    scanf("%d%d", &u, &v);
    u--, v--;
    nbrs[u].push_back(v);
    nbrs[v].push_back(u);
  }

  for (int i = 0; i < n; i++) scanf("%d", cs + i);

  ps[0] = -1;
  for (int u = 0; u >= 0;) {
    vi &nbru = nbrs[u];
    int up = ps[u];

    if (cis[u] < nbru.size()) {
      int v = nbru[cis[u]++];
      if (v != up) {
	ps[v] = u;
	u = v;
      }
    }
    else {
      if (up >= 0) {
	ss[up] += ss[u];
	if (! cs[u]) {
	  ss[up]++;
	  cs[u] ^= 1, cs[up] ^= 1;
	}
      }
      u = up;
    }
  }

  printf("%d\n", cs[0] ? ss[0] : -1);
  return 0;
}