#include <bits/stdc++.h>
using namespace std;
const long long MOD=1e9+7;

template<uint_fast64_t Modulus> class modint{
    using u64=uint_fast64_t;
    public:
    u64 a;
    constexpr modint(const u64 x=0) noexcept:a(((x%Modulus)+Modulus)%Modulus){}
    constexpr u64 &value() noexcept{return a;}
    constexpr const u64 &value() const noexcept{return a;}
    constexpr modint &operator+=(const modint &rhs) noexcept{
        a+=rhs.a;
        if (a>=Modulus) a-=Modulus;
        return *this;
    }
    constexpr modint operator+(const modint &rhs) const noexcept{
        return modint(*this)+=rhs;
    }
    constexpr modint &operator++() noexcept{
        return ++a,*this;
    }
    constexpr modint operator++(int) noexcept{
        modint t=*this; return ++a,t;
    }
    constexpr modint &operator-=(const modint &rhs) noexcept{
        if (a<rhs.a) a+=Modulus;
        a-=rhs.a;
        return *this;
    }
    constexpr modint operator-(const modint &rhs) const noexcept{
        return modint(*this)-=rhs;
    }
    constexpr modint &operator--() noexcept{
        return --a,*this;
    }
    constexpr modint operator--(int) noexcept{
        modint t=*this; return --a,t;
    }
    constexpr modint &operator*=(const modint &rhs) noexcept{
        a=a*rhs.a%Modulus;
        return *this;
    }
    constexpr modint operator*(const modint &rhs) const noexcept{
        return modint(*this)*=rhs;
    }
    constexpr modint &operator/=(modint rhs) noexcept{
        u64 exp=Modulus-2;
        while(exp){
            if (exp&1) *this*=rhs;
            rhs*=rhs; exp>>=1;
        }
        return *this;
    }
    constexpr modint operator/(const modint &rhs) const noexcept{
        return modint(*this)/=rhs;
    }
    constexpr modint operator-() const noexcept{
        return modint(Modulus-a);
    }
    constexpr bool operator==(const modint &rhs) const noexcept{
        return a==rhs.a;
    }
    constexpr bool operator!=(const modint &rhs) const noexcept{
        return a!=rhs.a;
    }
    constexpr bool operator!() const noexcept{return !a;}
    friend constexpr modint pow(modint rhs,long long exp) noexcept{
        modint res{1};
        while(exp){
            if (exp&1) res*=rhs;
            rhs*=rhs; exp>>=1;
        }
        return res;
    }
    template<class T> friend constexpr modint operator+(T x,modint y) noexcept{
        return modint(x)+y;
    }
    template<class T> friend constexpr modint operator-(T x,modint y) noexcept{
        return modint(x)-y;
    }
    template<class T> friend constexpr modint operator*(T x,modint y) noexcept{
        return modint(x)*y;
    }
    template<class T> friend constexpr modint operator/(T x,modint y) noexcept{
        return modint(x)/y;
    }
    friend ostream &operator<<(ostream &s,const modint &rhs) noexcept{
        return s << rhs.a;
    }
    friend istream &operator>>(istream &s,modint &rhs) noexcept{
        u64 a; rhs=modint{(s >> a,a)}; return s;
    }
};

using mint=modint<MOD>;

struct UnionFind{
    int num;
    vector<int> par,rank;
    UnionFind(int n):num(n),par(n),rank(n,1){
        iota(par.begin(),par.end(),0);
    }
    int root(int x){
        return (par[x]==x?x:par[x]=root(par[x]));
    }
    bool merge(int x, int y){
        x=root(x),y=root(y);
        if (x==y) return false;
        if (rank[x]<rank[y]) swap(x,y);
        par[y]=x;
        rank[x]+=rank[y];
        --num;
        return true;
    }
    bool same(int x, int y){return root(x)==root(y);}
    int size(int x){return rank[root(x)];}
    int count(){return num;}
};

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N; cin >> N;
    vector<int> P(N);
    UnionFind uf(N);
    for (int i=0;i<N;++i){
        cin >> P[i]; --P[i];
        if (P[i]==-2) P[i]=-1;
        if (~P[i]) uf.merge(i,P[i]);
    }
    vector<int> sz,check(N,0);
    int need=0;
    for (int i=0;i<N;++i) if (!check[uf.root(i)]){
        ++need; check[uf.root(i)]=1;
    }
    for (int i=0;i<N;++i) if (P[i]<0){
        sz.emplace_back(uf.size(i));
    }
    int n=sz.size();
    mint ans=pow((mint)N-1,n)*(N-need+n);
    vector<vector<mint>> dp(n+1,vector<mint>(n+1,0));
    dp[0][0]=1;
    for (int i=0;i<n;++i){
        ans-=pow((mint)N-1,n-1)*(sz[i]-1);
        for (int j=0;j<n;++j){
            dp[i+1][j]+=dp[i][j]*(N-1);
            dp[i+1][j+1]+=dp[i][j]*sz[i]*max(1,j);
        }
    }
    for (int j=2;j<=n;++j) ans-=dp[n][j];
    cout << ans << '\n';
}

Measured memory_bytes: 199092