#include <bits/stdc++.h>
using namespace std;
template <class T>
ostream& operator<<(ostream& stream, const vector<T> v) {
  stream << "[ ";
  for (int i = 0; i < (int)v.size(); i++) stream << v[i] << " ";
  stream << "]";
  return stream;
}
long long fpow(long long x, long long p, long long m) {
  long long r = 1;
  for (; p; p >>= 1) {
    if (p & 1) r = r * x % m;
    x = x * x % m;
  }
  return r;
}
long long inv(long long a, long long b) {
  return a < 2 ? a : ((a - inv(b % a, a)) * b + 1) / a % b;
}
int gcd(int a, int b) {
  if (!b) return a;
  return gcd(b, a % b);
}
long long gcd(long long a, long long b) {
  if (!b) return a;
  return gcd(b, a % b);
}
long long N, s[100010];
vector<int> g[100010];
long long res = 0;
bool ok = 1;
void dfs(int nod, int h, long long v) {
  if (h % 2) {
    if (s[nod] < v) ok = 0;
    res += s[nod] - v;
    for (int x : g[nod]) dfs(x, h + 1, s[nod]);
  } else {
    if (g[nod].size() == 0) return;
    long long minv = 1ll << 60;
    for (int x : g[nod]) minv = min(minv, s[x]);
    if (minv < v) ok = 0;
    res += minv - v;
    for (int x : g[nod]) dfs(x, h + 1, minv);
  }
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin >> N;
  int i;
  for (i = 2; i <= N; i++) {
    int p;
    cin >> p;
    g[p].push_back(i);
  }
  for (i = 1; i <= N; i++) cin >> s[i];
  dfs(1, 1, 0);
  if (!ok)
    cout << -1 << "\n";
  else
    cout << res << "\n";
}
