#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int n;
  cin >> n;
  vector<long long int> p(n, -1);
  map<long long int, vector<long long int> > c;
  c[0] = vector<long long int>();
  vector<long long int> s(n, 0);
  for (long long int x = 1; x < n; x++) {
    c[x] = vector<long long int>();
    long long int n;
    cin >> n;
    n--;
    p[x] = n;
    c[n].push_back(x);
  }
  for (long long int x = 0; x < n; x++) {
    cin >> s[x];
  }
  for (long long int x = n - 1; x >= 0; x--) {
    if (s[x] == -1) {
      long long int mn = INT_MAX;
      for (long long int i = 0; i < ((int)(c[x]).size()); i++) {
        mn = min(mn, s[c[x][i]]);
      }
      if (mn == INT_MAX) {
        mn = s[p[x]];
      }
      s[x] = mn;
    }
  }
  long long int sum = s[0];
  for (long long int i = 1; i < n; i++) {
    if (s[i] < s[p[i]]) {
      cout << -1 << endl;
      return 0;
    }
    sum = sum + s[i] - s[p[i]];
  }
  cout << sum << endl;
  return 0;
}
