#include <bits/stdc++.h>
using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<double, double> pdd;
//typedef vector<vector<ll>> Graph;

const ll mod = 1e9 + 7;
//const ll mod = 998244353;

#define REP(i,n) for(ll i=0;i<(ll)n;i++)
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define spa << " " <<
#define fi first
#define se second

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

template<class S, class T> ostream& operator << (ostream& os, const pair<S, T> v){
os << "(" << v.first << ", " << v.second << ")"; return os;
}
template<class T> ostream& operator << (ostream& os, const vector<T> v){
for(int i = 0; i < (int)v.size(); i++){if(i > 0){os << " ";} os << v[i];} return os;
}
template<class T> ostream& operator << (ostream& os, const vector<vector<T>> v){
for(int i = 0; i < (int)v.size(); i++){if(i > 0){os << endl;} os << v[i];} return os;
}

template<typename T> void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cerr<<v[i][0];for(ll j=1;j<w;j++)cerr spa v[i][j];cerr<<endl;}};
template<typename T> void debug(vector<T>&v,ll n){if(n!=0)cerr<<v[0];
for(ll i=1;i<n;i++)cerr spa v[i];
cerr<<endl;};

string num2bit(ll num, ll len){
    string bit = "";
    REP(i, len){
    bit += char('0'+(num>>i & 1));
    }
    return bit;
}

// par[i]  は，iがrootなら-(連結成分のサイズ)を，iがrootではないならばrootを返す
// root(x) は必ずroot を返す
struct UnionFindSize {
    vector<int> par;
    
    UnionFindSize(int n) : par(n, -1) { }
    void init(int n) { par.assign(n, -1); }
    
    int root(int x) {
        if (par[x] < 0) return x;
        else return par[x] = root(par[x]);
    }
    
    bool issame(int x, int y) {
        return root(x) == root(y);
    }
    
    bool merge(int x, int y) {
        x = root(x); y = root(y);
        if (x == y) return false;
        if (par[x] > par[y]) swap(x, y); // merge technique
        par[x] += par[y];
        par[y] = x;
        return true;
    }
    
    int size(int x) {
        return -par[root(x)];
    }

    void print(){
		cout << "uf: ";
		for(int i=0;i<(int)par.size();i++) cout << root(i) << " ";
		cout << endl;
	}
};
template <std::uint_fast64_t Modulus> class modint {
  // long long から modint を作るときは必ず正の数にしてからコンストラクタに入れること！ 
  // そうしないとバグります
  using u64 = std::uint_fast64_t;

public:
  u64 a;

  constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
  constexpr u64 &value() noexcept { return a; }
  constexpr const u64 &value() const noexcept { return a; }
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  constexpr modint &operator/=(modint rhs) noexcept {
    u64 exp = Modulus - 2;
    while (exp) {
      if (exp % 2) {
        *this *= rhs;
      }
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};
using mint = modint<mod>;

template <class T, class U> constexpr T power(T x, U exp) {
  T ret = static_cast<T>(1);
  while (exp) {
    if (exp % static_cast<U>(2) == static_cast<U>(1))
      ret *= x;
    exp /= static_cast<U>(2);
    x *= x;
  }
  return ::std::move(ret);
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    const int n_max = 5050;
    std::vector<mint> fact(n_max);
    fact[0] = 1;
    for (int i = 1; i < n_max; ++i) {
        fact[i] = fact[i - 1] * i;
    }

    ll N;
    cin >> N;
    vector<ll> P(N);
    REP(i, N) cin >> P[i];
    ll K = 0;
    REP(i, N){
        if(P[i] != -1) P[i]--;
        else K++;
    }

    UnionFindSize uf(N);
    REP(i, N) if(P[i]!=-1) uf.merge(i, P[i]);
    set<ll> st;
    REP(i, N) st.insert(uf.root(i));

    vector<ll> X;
    REP(i, N){
        if(P[i]==-1){
            X.push_back(uf.size(i));
            st.erase(uf.root(i));
        }
    }

    ll Y = st.size();

    mint res = 0;
    res += power(mint(N-1), K) * N;
    res -= power(mint(N-1), K) * Y;

    vector<vector<mint>> dp(K+1, vector<mint>(K+1, mint(0)));
    dp[0][0] = 1;
    REP(i, K){
        REP(j, K){
            dp[i+1][j] += dp[i][j];
            dp[i+1][j+1] += dp[i][j] * X[i];
        }
    }

    for(int i=1;i<=K;i++){
        if(i>1) res -= dp[K][i] * fact[i-1] * power(mint(N-1), K-i);
        else res -= (dp[K][i]-K) * fact[i-1] * power(mint(N-1), K-i);
    }

    cout << res.value() << endl;

    return 0;
}

Measured memory_bytes: 198988