#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <cmath>
#include <complex>
#include <cstring>
#include <deque>
#include <functional>
#include <iostream>
#include <iomanip>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#include <cstdint>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;
#define MP make_pair
#define PB push_back
#define inf 1000000007
#define rep(i,n) for(int i = 0; i < (int)(n); ++i)
#define all(x) (x).begin(),(x).end()

template<typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val){
    std::fill( (T*)array, (T*)(array+N), val );
}
 
template<class T> inline bool chmax(T &a, T b){
    if(a<b){
        a = b;
        return true;
    }
    return false;
}

template<class T> inline bool chmin(T &a, T b){
    if(a>b){
        a = b;
        return true;
    }
    return false;
}

template <unsigned int mod>
class ModInt {
private:
    unsigned int v;
    static unsigned int norm(const unsigned int& x){ return x < mod ? x : x - mod; }
    static ModInt make(const unsigned int& x){ ModInt m; return m.v = x, m; }
    static ModInt inv(const ModInt& x){ return make(inverse(x.v, mod)); }
    static unsigned int inverse(int a, int m){
        int u[] = {a, 1, 0}, v[] = {m, 0, 1}, t;
        while(*v){
            t = *u / *v;
            swap(u[0] -= t * v[0], v[0]), swap(u[1] -= t * v[1], v[1]), swap(u[2] -= t * v[2], v[2]);
        }
        return (u[1] % m + m) % m;
    }

public:
    ModInt() : v{0}{}
    ModInt(const long long val) : v{norm(val % mod + mod)} {}
    ModInt(const ModInt<mod>& n) : v{n()} {}
    explicit operator bool() const noexcept { return v != 0; }
    bool operator!() const noexcept { return !static_cast<bool>(*this); }
    ModInt& operator=(const ModInt& n){ return v = n(), (*this); }
    ModInt& operator=(const long long val){ return v = norm(val % mod + mod), (*this); }
    ModInt operator+() const { return *this; }
    ModInt operator-() const { return v == 0 ? make(0) : make(mod - v); }
    ModInt operator+(const ModInt& val) const { return make(norm(v + val())); }
    ModInt operator-(const ModInt& val) const { return make(norm(v + mod - val())); }
    ModInt operator*(const ModInt& val) const { return make((long long)v * val() % mod); }
    ModInt operator/(const ModInt& val) const { return *this * inv(val); }
    ModInt& operator+=(const ModInt& val){ return *this = *this + val; }
    ModInt& operator-=(const ModInt& val){ return *this = *this - val; }
    ModInt& operator*=(const ModInt& val){ return *this = *this * val; }
    ModInt& operator/=(const ModInt& val){ return *this = *this / val; }
    ModInt operator+(const long long val) const { return ModInt{v + val}; }
    ModInt operator-(const long long val) const { return ModInt{v - val}; }
    ModInt operator*(const long long val) const { return ModInt{(long long)v * (val % mod)}; }
    ModInt operator/(const long long val) const { return ModInt{(long long)v * inv(val)}; }
    ModInt& operator+=(const long long val){ return *this = *this + val; }
    ModInt& operator-=(const long long val){ return *this = *this - val; }
    ModInt& operator*=(const long long val){ return *this = *this * val; }
    ModInt& operator/=(const long long val){ return *this = *this / val; }
    bool operator==(const ModInt& val) const { return v == val.v; }
    bool operator!=(const ModInt& val) const { return !(*this == val); }
    bool operator==(const long long val) const { return v == norm(val % mod + mod); }
    bool operator!=(const long long val) const { return !(*this == val); }
    unsigned int operator()() const { return v; }
    friend ModInt operator+(const long long val, const ModInt& n) { return n + val; }
    friend ModInt operator-(const long long val, const ModInt& n) { return ModInt{val - n()}; }
    friend ModInt operator*(const long long val, const ModInt& n) { return n * val; }
    friend ModInt operator/(const long long val, const ModInt& n) { return ModInt{val} / n; }
    friend bool operator==(const long long val, const ModInt& n) { return n == val; }
    friend bool operator!=(const long long val, const ModInt& n) { return !(val == n); }
    friend istream& operator>>(istream& is, ModInt& n){
        unsigned int v;
        return is >> v, n = v, is;
    }
    friend ostream& operator<<(ostream& os, const ModInt& n){ return (os << n()); }
    friend ModInt mod_pow(ModInt x, long long n){
        ModInt ans = 1;
        while(n){
            if(n & 1) ans *= x;
            x *= x, n >>= 1;
        }
        return ans;
    }
};

#define MOD 1000000007
using mod = ModInt<MOD>;

#define MAX_N 200000
mod inv[MAX_N],fac[MAX_N],finv[MAX_N];

void make()
{
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for(int i=2;i<MAX_N;i++){
        inv[i] = MOD - inv[MOD % i] * (MOD / i);
        fac[i] = fac[i-1] * i;
        finv[i] = finv[i-1] * inv[i];
    }
}

mod comb(int a, int b)
{
    if(a<b) return 0;
    return fac[a] * finv[b] * finv[a-b];
}

class UnionFind {
private:
    int sz;
    vector<int> par, size_;
public:
    UnionFind(){}
    UnionFind(int node_size) : sz(node_size), par(sz), size_(sz, 1){
        iota(par.begin(), par.end(), 0);
    }
    int find(int x){
        if(par[x] == x) return x;
        else return par[x] = find(par[x]);
    }
    void unite(int x,int y){
        x = find(x), y = find(y);
        if(x == y) return;
        if(size_[x] < size_[y]) swap(x,y);
        par[y] = x;
        size_[x] += size_[y];
    }
    int size(int x){
        x = find(x);
        return size_[x];
    }
    bool same(int x,int y){
        return find(x) == find(y);
    }
};

mod dp[5010][5010];
int main(){
    make();
    int n;
    cin >> n;
    vector<int> a(n);
    UnionFind uf(n);
    int k = 0;
    rep(i,n){
        cin >> a[i];
        if(a[i]!=-1){
            a[i]--;
            uf.unite(i,a[i]);
        }else{
            k++;
        }
    }
    vector<bool> flag(n),p(n);
    rep(i,n){
        flag[uf.find(i)]= 1;
        if(a[i]==-1){
            p[uf.find(i)] = 1;
        }
    }
    int sm = 0;
    rep(i,n){
        if(flag[i])sm++;
    }
    int K = n-sm;
    if(k==0){
        cout << K << endl;
        return 0;
    }
    mod res = 0;
    // cerr << K << endl;
    // cerr << k << endl;
    res += (mod)(K)*mod_pow((mod)(n-1),k);
    // cerr << res << endl;
    mod P = mod_pow((mod)(n-1),k-1);
    mod Q = 0;
    if(k>=2) Q = mod_pow((mod)(n-1),k-2);
    rep(i,n){
        rep(j,i){
            if(flag[i]&&flag[j]&&i!=j){
                int si = uf.size(i);
                int sj = uf.size(j);
                if(p[i]){
                    res += (mod)(sj)*P;
                }
                if(p[j]){
                    res += (mod)(si)*P;
                }
                // if(p[i]&&p[j]){
                //     res -= (mod)(n-si)*(mod)(n-sj)*Q;
                // }
            }
        }
    }
    // cerr << res << endl;
    vector<int> s;
    rep(i,n){
        if(flag[i]&&p[i]){
            s.push_back(uf.size(i));
        }
    }
    int m = s.size();
    dp[0][0] = 1;
    for(int i=0;i<m;i++){
        for(int j=0;j<m;j++){
            dp[i+1][j]+= dp[i][j];
            dp[i+1][j+1] += dp[i][j]*(mod)(s[i]);
        }
    }
    // for(int i=1;i<=5;i++){
    //     cerr << dp[m][i] << endl;
    // }
    for(int i=2;i<=m;i++){
        // cerr << i << " " << dp[m][i] << " " << dp[m][i]*fac[i-1]*mod_pow(mod(n-1),k-i) << endl;
        res -= dp[m][i]*fac[i-1]*mod_pow(mod(n-1),k-i);
    }
    cout << res << endl;
    return 0;
}

Measured memory_bytes: 103376