#include <bits/stdc++.h>
using namespace std;
long long int XpowerY(long long int x, long long int y, long long int m) {
  long long int ans = 1;
  x = x % m;
  while (y > 0) {
    if (y % 2 == 1) ans = (ans * x) % m;
    x = ((x % m) * (x % m)) % m;
    y = y >> 1;
  }
  return ans % m;
}
vector<long long int> gr[100023];
long long int s[100023], val[100023];
long long int fl, minP;
long long int dfs(long long int node, long long int prev, long long int lev) {
  long long int mx = 1e12;
  if (lev % 2 == 0) {
    for (int i = 0; i < gr[node].size(); i++) {
      if (s[gr[node][i]] < prev) fl = 1;
      mx = min(mx, s[gr[node][i]] - prev);
    };
    val[node] = mx;
    if (gr[node].size() == 0) val[node] = 0;
  }
  for (int i = 0; i < gr[node].size(); i++) {
    val[gr[node][i]] = s[gr[node][i]] - mx - prev;
    long long int temp;
    if (lev % 2 == 0)
      temp = prev + val[node];
    else
      temp = s[node];
    dfs(gr[node][i], temp, lev + 1);
  }
  return 0;
}
int main() {
  long long int n, x;
  cin >> n;
  for (long long int i = 0; i < n - 1; i++) {
    cin >> x;
    gr[x].push_back(i + 2);
  }
  for (long long int i = 1; i <= n; i++) cin >> s[i];
  dfs(1, -1, 1);
  val[1] = s[1];
  if (fl) return cout << -1 << endl, 0;
  for (long long int i = 0; i < n; i++) {
    ;
    minP += val[i + 1];
  }
  cout << minP << endl;
  return 0;
}
