#include <stdio.h>

typedef struct list {
  int v;
  struct list *n;
} list;

int func (int v, list **e, int *c, int *visited) {
  list *l = e[v];
  int ans = 0;
  
  visited[v] = 1;
  while (l != NULL) {
    if (visited[l->v] <= 0) {
      ans += func(l->v, e, c, visited);
      if (c[l->v] <= 0) {
        ans++;
        c[l->v] = 1;
        c[v] = 1-c[v];
      }
    }
    l = l->n;
  }
  
  return ans;
}

int main () {
  int n = 0;
  int a = 0;
  int b = 0;
  int c[200000] = {};
  
  int res = 0;
  
  int ans = 0;
  int visited[200000] = {};
  
  list pool[399998] = {};
  int used = 0;
  
  list *e[200000] = {};
  
  res = scanf("%d", &n);
  for (int i = 0; i < n-1; i++) {
    res = scanf("%d", &a);
    res = scanf("%d", &b);
    a--;
    b--;
    pool[used].v = b;
    pool[used].n = e[a];
    e[a] = pool+used;
    used++;
    pool[used].v = a;
    pool[used].n = e[b];
    e[b] = pool+used;
    used++;
  }
  for (int i = 0; i < n; i++) {
    res = scanf("%d", c+i);
  }
  
  ans = func(0, e, c, visited);
  
  if (c[0] > 0) {
    printf("%d\n", ans);
  } else {
    printf("-1\n");
  }
  
  return 0;
}