#include <bits/stdc++.h>
const int N = 2e5 + 5;
using namespace std;
vector<int> G[N];
long long b[N];
int vis[N];
long long ans[N];
long long flag = 1;
void dfs(int x, int fa) {
  long long minn = 0x3f3f3f3f;
  if (b[x] == -1) {
    for (int i = 0; i < G[x].size(); i++) {
      int g = G[x][i];
      if (!vis[g]) {
        vis[g] = 1;
        minn = min(minn, b[g]);
        dfs(g, x);
      }
      if (minn != 0x3f3f3f3f)
        b[x] = minn;
      else
        b[x] = b[fa];
    }
  } else {
    for (int i = 0; i < G[x].size(); i++) {
      int g = G[x][i];
      if (!vis[g]) {
        vis[g] = 1;
        dfs(g, x);
      }
    }
  }
}
void dfs2(int x, int fa) {
  for (int i = 0; i < G[x].size(); i++) {
    int g = G[x][i];
    if (!vis[g]) {
      vis[g] = 1;
      ans[g] = b[g] - b[x];
      if (ans[g] < 0) {
        flag = 0;
        return;
      }
      dfs2(g, x);
    }
  }
}
int main() {
  int n;
  cin >> n;
  for (int i = 2; i <= n; i++) {
    int u;
    cin >> u;
    G[i].push_back(u);
    G[u].push_back(i);
  }
  for (int i = 1; i <= n; i++) cin >> b[i];
  vis[1] = 1;
  dfs(1, -1);
  memset(vis, 0, sizeof vis);
  vis[1] = 1;
  ans[1] = b[1];
  dfs2(1, -1);
  if (flag) {
    long long res = 0;
    for (int i = 1; i <= n; i++)
      if (ans[i] > 0) res += ans[i];
    cout << res << endl;
  } else
    cout << -1 << endl;
}
