#include <bits/stdc++.h>
using namespace std;
const long long N = 200001;
std::vector<long long> g[N];
long long s[N];
long long h[N];
long long a[N] = {0};
bool visited[N] = {false};
void dfs1(long long n, long long parent, long long depth) {
  h[n] = depth;
  for (long long v : g[n]) {
    if (v != parent) {
      dfs1(v, n, depth + 1);
    }
  }
}
void dfs2(long long n, long long parent) {
  if (h[n] % 2 == 0) {
    for (long long v : g[n]) {
      if (v != parent) {
        a[n] = s[v] - s[parent];
      }
    }
    for (long long v : g[n]) {
      if (v != parent) {
        a[n] = min(s[v] - s[parent], a[n]);
      }
    }
    s[n] = a[n] + s[parent];
  } else {
    a[n] = s[n] - s[parent];
  }
  for (long long v : g[n]) {
    if (v != parent) {
      dfs2(v, n);
    }
  }
}
int main() {
  long long n;
  cin >> n;
  for (long long i = 2; i <= n; i++) {
    long long parent;
    cin >> parent;
    g[parent].push_back(i);
    g[i].push_back(parent);
  }
  for (long long i = 1; i <= n; i++) {
    cin >> s[i];
  }
  dfs1(1, -1, 1);
  a[1] = s[1];
  dfs2(1, n + 1);
  long long sum = 0;
  for (long long i = 1; i <= n; i++) {
    if (a[i] < 0) {
      cout << "-1" << endl;
      return 0;
    }
    sum += a[i];
  }
  cout << sum << endl;
}
