#include <bits/stdc++.h>
using namespace std;
const long long M = 1e5 + 10;
const long long inf = 1e18 + 7;
long long head[M], cnt, flag;
long long ans, a[M], b[M];
vector<long long> g[M];
void dfs(long long u, long long fa, long long dep) {
  long long mn = inf;
  if (dep % 2 == 0) {
    for (long long i = 0; i < g[u].size(); i++) {
      long long v = g[u][i];
      mn = min(mn, a[v]);
    }
    if (mn != inf)
      a[u] = mn, b[u] = a[u] - a[fa];
    else
      a[u] = a[fa], b[u] = 0;
  } else {
    b[u] = a[u] - a[fa];
  }
  for (long long i = 0; i < g[u].size(); i++) {
    long long v = g[u][i];
    dfs(v, u, dep + 1);
  }
}
int main() {
  long long n, x;
  scanf("%lld", &n);
  for (long long i = 2; i <= n; i++) {
    scanf("%lld", &x);
    g[x].push_back(i);
  }
  for (long long i = 1; i <= n; i++) {
    scanf("%lld", &a[i]);
  }
  dfs(1, 0, 1);
  for (long long i = 1; i <= n; i++) {
    ans += b[i];
    if (b[i] < 0) flag = 1;
  }
  if (!flag)
    printf("%lld\n", ans);
  else
    printf("-1\n");
}
