#include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <cstdio>
#include <cmath>
#include <tuple>
#include <iomanip>
#include <numeric>
#include <unordered_map>
#include <sstream>
#include<limits.h>
#include<float.h>
#include<list>
#include <array>
#include <complex>
#include<stdio.h>
#include<string.h>
#include <bitset>
#include<assert.h>

using namespace std;
#define int long long

#define I32_MAX 2147483647
#define I64_MAX 9223372036854775807LL
#define I64_MAX2 1223372036854775807LL
#define INF I64_MAX2
#define MOD 1000000007


#define MEM_SIZE 10000
#define DEBUG_OUT true
#define ALL(x) (x).begin(), (x).end()

template<typename T> void DEBUG(T e){if(DEBUG_OUT == false)return; std::cout << e <<" ";}
template<typename T> void DEBUG(const std::vector<T>& v){if(DEBUG_OUT == false)return;for(const auto& e : v){std::cout<< e << " "; } std::cout << std::endl;}
template<typename T> void DEBUG(const std::vector<std::vector<T> >& vv){if(DEBUG_OUT == false)return;for(const auto& v : vv){ DEBUG(v); } }
template<class T,class... Ts> void DEBUG(T d, Ts... e){if(DEBUG_OUT == false)return;DEBUG(d);DEBUG(e...);}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; abort();}}
template< typename T1, typename T2 > inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template< typename T1, typename T2 > inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

struct UnionFind{
  int num;
  vector<int> rs,ps;
  UnionFind(){}
  UnionFind(int n):num(n),rs(n,1),ps(n,0){iota(ps.begin(),ps.end(),0);}
  int find(int x){
    return (x==ps[x]?x:ps[x]=find(ps[x]));
  }
  bool same(int x,int y){
    return find(x)==find(y);
  }
  void unite(int x,int y){
    x=find(x);y=find(y);
    if(x==y) return;
    if(rs[x]<rs[y]) swap(x,y);
    rs[x]+=rs[y];
    ps[y]=x;
    num--;
  }
  int size(int x){
    return rs[find(x)];
  }
  int count() const{
    return num;
  }
};

//BELOW
struct mint
{
  int x; // typedef long long int;
  mint(int x = 0) : x((x % MOD + MOD) % MOD) {}
  mint &operator+=(const mint a)
  {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint a)
  {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint a)
  {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const
  {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const
  {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const
  {
    mint res(*this);
    return res *= a;
  }
  mint pow(int t) const
  {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime MOD
  mint inv() const
  {
    return pow(MOD - 2);
  }
  mint &operator/=(const mint a)
  {
    return (*this) *= a.inv();
  }
  mint operator/(const mint a) const
  {
    mint res(*this);
    return res /= a;
  }
};

const int MAX = 510000;

long long fac[MAX], finv[MAX], inv[MAX];
// テーブルを作る前処理
void COMinit()
{
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++)
  {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

int COM(int n, int k)
{
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

int PERM(int n, int k)
{
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[n - k] % MOD) % MOD;
}

int MCOM(int n, int k)
{
  //n個のものから重複を許してr個選ぶ。
  //Choose R from N types of things, allowing for duplication.
  return COM(n + k - 1, k);
}
//solve

//-----CODE------//



void solve(void)
{
  int N;cin>>N;
  vector<int> vec (N,0);
  int mcnt = 0;

  for (int i = 0; i < N; i++)
  {
     cin>>vec[i];
     vec[i]--;
     if(vec[i] == - 2) 
     {
       vec[i]++;
       mcnt++;
     }
  }
  UnionFind DS(N);

  int cnt = 0;
  
  for (int i = 0; i < N; i++)
  {
    if(vec[i] >= 0)
    {
        DS.unite(i,vec[i]);
      
    }
  }
  vector<int> vs;
  for (int i = 0; i < N; i++)
  {
    if(vec[i] >= 0)continue;
    vs.emplace_back(DS.size(i));

  }
  int k = vs.size();
  COMinit();
  vector<mint> po(k+1,1);
  for (int i = 0; i < k; i++)
  {
    po[i+1] = po[i]*(mint)(N-1);
  }

  mint res = 0;

  res += (mint)(DS.count()-k)*po[k];

  vector<mint> DP(k+2,0);
  DP[0] = (mint)1;
  for (auto &&v : vs)// 1 12 123 1234 
  {
    res += (mint)(v-1)*po[k-1];
    vector<mint> nx(k+2,0);
    for (int j = 0; j < k; j++)
    {
      nx[j+0] += DP[j];
      nx[j+1] += DP[j]*(mint)(v);

      if(1 <= j and j+1 <= k)
      res += DP[j]*(mint)(v)*(mint)(fac[j])*po[k-(j+1)];
    }
    swap(DP,nx);
  }
  
  mint ANS = po[k]*mint(N) - res;
  cout<<ANS.x<<endl;
  
  
  return;
}
int32_t main(int32_t argc, const char *argv[])
{
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);

  std::cout << std::fixed;
  std::cout << std::setprecision(11);
  solve();

  return 0;
}


Measured memory_bytes: 15836