#pragma region template
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
 
using ll=long long;
using ld=long double;
using vi=vector<int>;
using vll=vector<ll>;
using pi=pair<int,int>;
using pll=pair<ll,ll>;
 
#define overload2(a,b,c,...) c
#define overload3(a,b,c,d,...) d
#define overload4(a,b,c,d,e,...) e
#define overload5(a,b,c,d,e,f,...) f
 
#define TYPE1(T) template<typename T>
#define TYPE2(T,U) template<typename T,typename U>
#define TYPE(...) overload2(__VA_ARGS__,TYPE2,TYPE1)(__VA_ARGS__)
#define TYPES1(T) template<typename... T>
#define TYPES2(H,T) template<typename H,typename... T>
#define TYPES(...) overload2(__VA_ARGS__,TYPES2,TYPES1)(__VA_ARGS__)
 
#define REP4(i,s,n,d) for(int i=(s);i<(n);i+=(d))
#define REP3(i,s,n) REP4(i,s,n,1)
#define REP2(i,n) REP3(i,0,n)
#define REP1(n) REP2(tomato,n)
#define REP(...) overload4(__VA_ARGS__,REP4,REP3,REP2,REP1)(__VA_ARGS__)
 
#define RREP4(i,n,s,d) for(int i=(n)-1;i>=(s);i-=(d))
#define RREP3(i,n,s) RREP4(i,n,s,1)
#define RREP2(i,n) RREP3(i,n,0)
#define RREP1(n) RREP2(tomato,n)
#define RREP(...) overload4(__VA_ARGS__,RREP4,RREP3,RREP2,RREP1)(__VA_ARGS__)

#define FOR4(a,b,c,d,v) for(auto [a,b,c,d]:v)
#define FOR3(a,b,c,v) for(auto [a,b,c]:v)
#define FOR2(a,b,v) for(auto [a,b]:v)
#define FOR1(a,v) for(auto a:v)
#define FOR(...) overload5(__VA_ARGS__,FOR4,FOR3,FOR2,FOR1)(__VA_ARGS__)

#define AFOR4(a,b,c,d,v) for(auto&[a,b,c,d]:v)
#define AFOR3(a,b,c,v) for(auto&[a,b,c]:v)
#define AFOR2(a,b,v) for(auto&[a,b]:v)
#define AFOR1(a,v) for(auto&a:v)
#define AFOR(...) overload5(__VA_ARGS__,AFOR4,AFOR3,AFOR2,AFOR1)(__VA_ARGS__)

#define CFOR4(a,b,c,d,v) for(const auto&[a,b,c,d]:v)
#define CFOR3(a,b,c,v) for(const auto&[a,b,c]:v)
#define CFOR2(a,b,v) for(const auto&[a,b]:v)
#define CFOR1(a,v) for(const auto&a:v)
#define CFOR(...) overload5(__VA_ARGS__,CFOR4,CFOR3,CFOR2,CFOR1)(__VA_ARGS__)
 
#define ALL(v) v.begin(),v.end()
#define RALL(v) v.rbegin(),v.rend()
#define SORT(v) sort(ALL(v))
#define RSORT(v) sort(RALL(v))
#define REVERSE(v) reverse(ALL(v))
#define UNIQUE(v) SORT(v),v.erase(unique(ALL(v)),v.end())
#define SZ(v) int(v.size())
 
TYPES(T) void input(T&... a){ (cin>>...>>a); }
#define DECLARE(T,...) T __VA_ARGS__;input(__VA_ARGS__);
#define INT(...) DECLARE(int,__VA_ARGS__)
#define STR(...) DECLARE(string,__VA_ARGS__)
#define LL(...) DECLARE(long long,__VA_ARGS__)
#define CHR(...) DECLARE(char,__VA_ARGS__)
#define DBL(...) DECLARE(double,__VA_ARGS__)
#define VI(n,v) vi v(n);cin>>v;
#define VLL(n,v) vll v(n);cin>>v;
 
TYPE(T) istream&operator>>(istream&is,vector<T>&v){
  for(auto&a:v)cin>>a;
  return is;
}
TYPE(T) ostream&operator<<(ostream&os,const vector<T>&v){
  if(&os==&cerr)os<<"[";
  REP(i,v.size()){
    os<<v[i];
    if(i+1<v.size())os<<(&os==&cerr?",":" ");
  }
  if(&os==&cerr)os<<"]";
  return os;
}
TYPE(T,S) istream&operator>>(istream&is,pair<T,S>&p){
  cin>>p.first>>p.second;
  return is;
}

#ifdef __DEBUG
 #include <debug>
#else
 #define debug(...) void(0)
#endif

void print(){ cout << '\n'; }
TYPES(T,Ts) void print(const T& a,const Ts&... b){
  cout<<a;
  (cout<<...<<(cout<< ' ',b));
  cout << '\n';
}
 
TYPE(T) using pq=priority_queue<T>;
TYPE(T) using pqg=priority_queue<T,vector<T>,greater<T>>;
TYPE(T) T pick(queue<T>& que){assert(que.size()); T a=que.front();que.pop();return a;}
TYPE(T) T pick(pq<T>& que){assert(que.size()); T a=que.top();que.pop();return a;}
TYPE(T) T pick(pqg<T>& que){assert(que.size()); T a=que.top();que.pop();return a;}
TYPE(T) T pick(stack<T>& sta){assert(sta.size()); T a=sta.top();sta.pop();return a;}
 
string YES(bool f=true){return (f?"YES":"NO");}
string Yes(bool f=true){return (f?"Yes":"No");}
string yes(bool f=true){return (f?"yes":"no");}
 
constexpr int INF=1e9+7;
constexpr ll LINF=ll(1e18)+7;
constexpr ld EPS=1e-10;
 
vi iota(int n){vi a(n);iota(ALL(a),0);return a;}
TYPE(T) vector<pair<T,int>> query_sort(const vector<T>&v){
  vector<pair<T,int>> res(v.size());
  REP(i,v.size())res[i]={v[i],i};
  SORT(res);
  return res;
}
TYPE(T) T rev(T a){ REVERSE(a);return a; }
TYPE(T) void fin(T a){cout<<a<<'\n';exit(0);}
TYPE(T) bool chmax(T &a,T b){return (a<b&&(a=b,true));}
TYPE(T) bool chmin(T &a,T b){return (a>b&&(a=b,true));}
TYPES(T,Ns) auto make_vector(T x,int n,Ns ...ns){
  if constexpr(sizeof...(ns)==0)return vector<T>(n,x);
  else return vector(n,make_vector<T>(x,ns...));
}
bool in(const ll S,const int a){return (S>>a)&1;}
int popcount(const ll S){return __builtin_popcountll(S);}
int digit(char c){ return (c>='0' and c<='9' ? c-'0' : -1);}
ll sqrtll(ll a){ 
  for(ll b=sqrt(a);b*b<=a;b++)if(b*b==a)return b;
  for(ll b=sqrt(a);b>=0 and b*b>=a;b--)if(b*b==a)return b;
  return -1;
}
#pragma endregion template

struct Edge{
  int from,to;
  Edge()=default;
  Edge(int from,int to):from(from),to(to){}
};

struct Graph{
  int n;
  using edge_type=Edge;
  vector<edge_type> edges;
protected:
  vector<int> in_deg;
  bool prepared;
  class OutgoingEdges{
    Graph* g;
    int l,r;
  public:
    OutgoingEdges(Graph* g,int l,int r):g(g),l(l),r(r){}
    edge_type* begin(){ return &(g->edges[l]); }
    edge_type* end(){ return &(g->edges[r]); }
    edge_type& operator[](int i){ return g->edges[l+i]; }
    int size()const{ return r-l; }
  };
  class ConstOutgoingEdges{
    const Graph* g;
    int l,r;
  public:
    ConstOutgoingEdges(const Graph* g,int l,int r):g(g),l(l),r(r){}
    const edge_type* begin()const{ return &(g->edges[l]); }
    const edge_type* end()const{ return &(g->edges[r]); }
    const edge_type& operator[](int i)const{ return g->edges[l+i]; }
    int size()const{ return r-l; }
  };
public:
  OutgoingEdges operator[](int v){
    assert(prepared);
    return { this,in_deg[v],in_deg[v+1] };
  }
  const ConstOutgoingEdges operator[](int v)const{
    assert(prepared);
    return { this,in_deg[v],in_deg[v+1] };
  }

  bool is_prepared()const{ return prepared; }

  Graph():n(0),in_deg(1,0),prepared(false){}
  Graph(int n):n(n),in_deg(n+1,0),prepared(false){}
  Graph(int n,int m,bool directed=false,int indexed=1):
    n(n),in_deg(n+1,0),prepared(false){ scan(m,directed,indexed); }

  void resize(int n){n=n;}

  void add_arc(int from,int to){
    assert(!prepared);
    assert(0<=from and from<n and 0<=to and to<n);
    edges.emplace_back(from,to);
    in_deg[from+1]++;
  }
  void add_edge(int u,int v){
    add_arc(u,v);
    add_arc(v,u);
  }
  void add_arc(const edge_type&e){
    add_arc(e.from,e.to);
  }
  void add_edge(const edge_type&e){
    add_edge(e.from,e.to);
  }

  void scan(int m,bool directed=false,int indexed=1){
    edges.reserve(directed?m:2*m);
    while(m--){
      int u,v;cin>>u>>v;u-=indexed;v-=indexed;
      if(directed)add_arc(u,v);
      else add_edge(u,v);
    }
    build();
  }

  void build(){
    assert(!prepared);prepared=true;
    for(int v=0;v<n;v++)in_deg[v+1]+=in_deg[v];
    vector<edge_type> new_edges(in_deg.back());
    auto counter=in_deg;
    for(auto&&e:edges)new_edges[ counter[e.from]++ ]=e;
    edges=new_edges;
  }

  void graph_debug()const{
  #ifndef __LOCAL
    return;
  #endif
    assert(prepared);
    for(int from=0;from<n;from++){
      cerr<<from<<";";
      for(int i=in_deg[from];i<in_deg[from+1];i++)
        cerr<<edges[i].to<<" ";
      cerr<<"\n";
    }
  }
};
struct Tree:Graph{
  using Graph::Graph;
  int root=-1;
  vector<int> DFS,BFS,depth;

  void scan_root(int indexed=1){
    for(int i=1;i<n;i++){
      int p;cin>>p;
      add_edge(p-indexed,i);
    }
    build();
  }
  void scan(int indexed=1){
    Graph::scan(n-1,false,indexed);
    build();
  }

  edge_type& parent(int v){
    assert(~root and root!=v);
    return (*this)[v][0];
  }
  OutgoingEdges son(int v){
    assert(~root);
    if(v==root)return {this,in_deg[v],in_deg[v+1]};
    return {this,in_deg[v]+1,in_deg[v+1]};
  }

private:
  void dfs(int v,int pre=-1){
    for(auto&e:(*this)[v]){
      if(e.to==pre)swap((*this)[v][0],e);
      else{
        depth[e.to]=depth[v]+1;
        dfs(e.to,v);
      }
    }
    DFS.push_back(v);
  }
public:
  void build(int r=0){
    if(!is_prepared())Graph::build();
    if(~root){
      assert(r==root);
      return;
    }
    root=r;
    depth=vector<int>(n,0);
    DFS.reserve(n);BFS.reserve(n);
    dfs(root);
    queue<int> que;
    que.push(root);
    while(que.size()){
      int p=que.front();que.pop();
      BFS.push_back(p);
      for(const auto&e:son(p))que.push(e.to);
    }
  }
};

int main(){
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  INT(n);
  Tree t(n);
  t.scan();
  VI(n,c);

  t.build(0);

  int ans=0;
  CFOR(v,t.DFS){
    CFOR(e,t.son(v)){
      int u=e.to;
      if(!c[u]){
        ans++;
        c[v]^=1;
      }
    }
  }
  if(c[0])fin(ans);
  fin(-1);
}