#include <bits/stdc++.h>
const int N = 1e5 + 5;
const long double pi = 3.141592653;
const long long MOD = 1e9 + 7;
const long long mod = 998244353;
const long long INF = 9223372036854775807LL;
using namespace std;
int dx[9] = {1, -1, 0, 0, 1, 1, -1, -1, 0};
int dy[9] = {0, 0, 1, -1, 1, -1, 1, -1, 0};
vector<long long> g[N], s(N), a(N);
vector<bool> vis(N, false);
void dfs(long long t, long long sum) {
  vis[t] = true;
  if (s[t] != -1)
    a[t] = s[t] - sum;
  else {
    long long val = INF;
    for (long long i = 0; i < (long long)g[t].size(); i++)
      val = min(val, s[g[t][i]]);
    if (val != INF) {
      a[t] = val - sum;
      s[t] = sum + a[t];
    } else {
      s[t] = sum;
      a[t] = 0;
    }
  }
  for (long long i = 0; i < (long long)g[t].size(); i++) {
    if (!vis[g[t][i]]) dfs(g[t][i], sum + a[t]);
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int T = 1;
  while (T--) {
    long long n, i, x, ans = 0;
    cin >> n;
    for (i = 2; i <= n; i++) {
      cin >> x;
      g[x].push_back(i);
    }
    for (i = 1; i <= n; i++) cin >> s[i];
    dfs(1, 0);
    for (i = 1; i <= n; i++) {
      if (a[i] < 0)
        break;
      else
        ans = ans + a[i];
    }
    if (i <= n)
      cout << -1;
    else
      cout << ans;
  }
  return 0;
}
