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

template <int mod = (int)(1e9 + 7)>
struct ModInt {
  int x;
  ModInt() : x(0) {}
  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}
  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod) x -= mod;
    return *this;
  }
  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod) x -= mod;
    return *this;
  }
  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }
  ModInt operator-() const { return ModInt(-x); }
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }
  bool operator==(const ModInt &p) const { return x == p.x; }
  bool operator!=(const ModInt &p) const { return x != p.x; }
  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }
  ModInt pow(int64_t n) const {
    ModInt res(1), mul(x);
    while (n) {
      if (n & 1) res *= mul;
      mul *= mul;
      n >>= 1;
    }
    return res;
  }
  friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; }
  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt<mod>(t);
    return (is);
  }
  static int get_mod() { return mod; }
};

struct Combination {
  vector<ModInt<>> _fact, _rfact, _inv;
  Combination(long long nsize = 5000000)
      : _fact(nsize + 1), _rfact(nsize + 1), _inv(nsize + 1) {
    _fact[0] = _rfact[nsize] = _inv[0] = 1;
    for (int i = 1; i <= nsize; i++) _fact[i] = _fact[i - 1] * i;
    _rfact[nsize] /= _fact[nsize];
    for (int i = nsize - 1; i >= 0; i--) _rfact[i] = _rfact[i + 1] * (i + 1);
    for (int i = 1; i <= nsize; i++) _inv[i] = _rfact[i] * _fact[i - 1];
  }
  inline ModInt<> fact(int k) const { return _fact[k]; }

  inline ModInt<> rfact(int k) const { return _rfact[k]; }

  inline ModInt<> inv(int k) const { return _inv[k]; }

  ModInt<> P(int n, int r) const {
    if (r < 0 || n < r) return 0;
    return fact(n) * rfact(n - r);
  }

  ModInt<> C(int p, int q) const {
    if (q < 0 || p < q) return 0;
    return fact(p) * rfact(q) * rfact(p - q);
  }
  // n types,choose r
  ModInt<> H(int n, int r) const {
    if (n < 0 || r < 0) return (0);
    return r == 0 ? 1 : C(n + r - 1, r);
  }
};

struct Unionfind {
  // tree number
  vector<int> par;
  // constructor
  Unionfind(int n = 1) : par(n, -1) {}
  // search root
  int root(int x) {
    if (par[x] < 0) return x;
    return par[x] = root(par[x]);
  }
  // is same?
  bool issame(int x, int y) { return root(x) == root(y); }

  // add
  // already added, return 0
  bool uni(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y) return 0;
    if (par[x] > par[y]) swap(x, y);
    par[x] += par[y];
    par[y] = x;
    return 1;
  }
  int size(int x) { return -par[root(x)]; }
};

long long n, cnt = 0;
vector<long long> p, memo, sz;
Unionfind uf;
Combination com;

ModInt<> solve();

int main() {
  cin >> n;
  p.resize(n);
  for (int i = 0; i < n; ++i) {
    cin >> p[i];
    --p[i];
  }
  cout << solve() << endl;
  return 0;
}

ModInt<> solve() {
  ModInt<> res = n, now = n - 1;
  uf = Unionfind(n);
  sz.assign(n, -1);
  memo.assign(n, 0);
  for (int i = 0; i < n; ++i)
    if (p[i] >= 0)
      uf.uni(i, p[i]);
    else
      ++cnt;
  for (int i = 0; i < n; ++i) {
    if (sz[uf.root(i)] < 0) sz[uf.root(i)] = uf.size(i);
    if (p[i] < 0) ++memo[uf.root(i)];
  }
  vector<long long> v;
  vector<ModInt<>> dp;
  for (int i = 0; i < n; ++i)
    if (sz[i] > 0) {
      if (memo[i] > 0)
        v.push_back(sz[i]);
      else
        res -= 1;
    }
  res *= now.pow(cnt);
  n = v.size();
  dp.assign(n + 1, 0);
  dp[0] = 1;
  for (int i = 0; i < n; ++i)
    for (int j = n; j > 0; --j) dp[j] += dp[j - 1] * max(j - 1, 1) * v[i];
  for (int i = 2; i <= n; ++i) res -= dp[i] * now.pow(n - i);
  for (int i = 0; i < n; ++i) res -= now.pow(n - 1) * (v[i] - 1);
  return res;
}

Measured memory_bytes: 62248