#include <bits/stdc++.h>
 
using namespace std;
 
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl;
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
typedef long long ll;
#define int ll
 
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(int)(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
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; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const long double pi = 3.1415926535897932384626433832795028841971L;
#define Sp(p) cout<<setprecision(25)<< fixed<<p<<endl;
//int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
//int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
vi dx = {1, 0, -1, 0}, dy = {0, 1, 0, -1};
vi dx2 = { 1,1,0,-1,-1,-1,0,1 }, dy2 = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
const ll MOD = 1000000007;
// const ll MOD = 998244353;
// #define mp make_pair
//#define endl '\n'


class UnionFind {
public:
  int n;
  vi par; //�e
  vi ran; //�؂̐[��
  vi num; //�v�f��
  int g; // group��
 
  UnionFind(int _n) {
    n = _n;
    g = n;
    par.resize(n); ran.resize(n); num.resize(n);
    for (int i = 0; i < n; i++) {
      par[i] = i; ran[i] = 0; num[i] = 1;
    }
  }
 
  //�؂̍������߂�
  int find(int x) {
    if (par[x] == x) {
      return x;
    }
    else {
      return par[x] = find(par[x]);
    }
  }
 
  //x��y�̑�����W���𕹍�
  void unite(int x, int y) {
    x = find(x); y = find(y);
    int numsum = num[x] + num[y];
    if (x == y) {
      return;
    }
    if (ran[x]<ran[y]) {
      par[x] = y;
    }
    else {
      par[y] = x;
      if (ran[x] == ran[y]) {
        ran[x]++;
      }
    }
    num[x] = num[y] = numsum;
    g--;
  }
 
  //x��y�������W���ɑ����邩�ۂ�
  bool same(int x, int y) {
    return find(x) == find(y);
  }
 
};

const int MAXN = 555555;

vl fact(MAXN);
vl rfact(MAXN);

ll mod_pow(ll x, ll p, ll M = MOD) {
  if (p < 0) {
    x = mod_pow(x, M - 2, M);
    p = -p;
  }
  ll a = 1;
  while (p) {
    if (p % 2)
      a = a*x%M;
    x = x*x%M;
    p /= 2;
  }
  return a;
}

ll mod_inverse(ll a, ll M = MOD) {
  return mod_pow(a, M - 2, M);
}

void set_fact(ll n, ll M = MOD) {
  fact[0] = fact[1] = rfact[0] = rfact[1] = 1;
  for (ll i = 2; i <= n; i++) {
    fact[i] = i * fact[i - 1] % M;
    // rfact[i] = mod_inverse(fact[i], M);
  }
}

//http://drken1215.hatenablog.com/entry/2018/06/08/210000
//n���傫��fact���v�Z�ł��Ȃ��Ƃ��̂ق��̌v�Z���@�ɂ��ď����Ă���
ll nCr(ll n, ll r, ll M = MOD) {
  if (r > n) return 0;
  assert(fact[2] == 2);
  ll ret = fact[n];
  if (rfact[r] == 0) {
    rfact[r] = mod_inverse(fact[r], M);
  }
  ret = (ret*rfact[r]) % M;
  if (rfact[n - r] == 0) {
    rfact[n - r] = mod_inverse(fact[n - r], M);
  }
  ret = (ret*rfact[n - r]) % M;
  return ret;
}

ll nHr(ll n, ll r) {
  return nCr(n+r-1, r);
}

signed main() {
    fio();
    int n;
    cin >> n;
    set_fact(n + 10);
    vi p(n);
    UnionFind uf(n);
    int cnt = 0;
    rep (i, n) {
        cin >> p[i];
        if (p[i] != -1) {
            p[i]--;
            uf.unite(i, p[i]);
        }
        else {
            cnt++;
        }
    }

    vector<bool> hei(n, true);
    vector<int> nnum(n);
    rep (i, n) {
        int u = uf.find(i);
        nnum[u]++;
        if (p[i] == -1) hei[u] = false;
    }
    
    vi num;
    ll ans = 0;
    rep (i, n) {
        if (nnum[i] == 0) continue;
        if (hei[i]) {
            ans += mod_pow(n - 1, cnt);
        }
        else {
            num.push_back(nnum[i]);
        }
    }
    ans %= MOD;

    vll dp(num.size() + 1, vl(num.size() + 1));
    dp[0][0] = 1;
    rep (i, num.size()) {
        rep (j, num.size() + 1) {
            if (dp[i][j] == 0) continue;
            (dp[i + 1][j] += dp[i][j]) %= MOD;
            (dp[i + 1][j + 1] += dp[i][j] * num[i] % MOD) %= MOD;
        }
    }
    // DEBUG_MAT(dp);

    rep1 (j, num.size()) {
        (dp[num.size()][j] *= fact[j - 1]) %= MOD;
    }

    for (int i = 2; i <= num.size(); i++) {
        ans += dp.back()[i] * mod_pow(n - 1, cnt - i) % MOD;
    }
    for (int x: num) {
        ans += (x - 1) * mod_pow(n - 1, cnt - 1) % MOD;
    }
    ans %= MOD;

    ans = n * mod_pow(n - 1, cnt) % MOD - ans;
    ans = (ans % MOD + MOD) % MOD;

    cout << ans << endl;
}

Measured memory_bytes: 207632