Sum in the tree

#include<bits/stdc++.h>
const int maxn = 100100;
int n;
int fa[maxn];
int s[maxn],s2[maxn];
int main(){
    std::ios::sync_with_stdio(false),std::cin.tie(0);
    std::cin >> n;
    for(int i=2;i<=n;++i)std::cin >> fa[i];
    for(int i=1;i<=n;++i)std::cin >> s[i],s2[i]=s[i],s[i]<0?s[i]=2e9:0;
    for(int i=n;i!=1;--i)if(s[fa[i]]>s[i])s[fa[i]]=s[i];
    long long sum=0;
    for(int i=1;i<=n;++i)if(s2[i]>=0&&s2[i]!=s[i])return std::cout << -1 << ‘\n‘,0;
    else if(s[i]<=1e9)sum+=s[i]-s[fa[i]];
    std::cout << sum << ‘\n‘;
}