#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < int(n); i++)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define reps(i, n) for (int i = 1; i <= int(n); i++)
#define rreps(i, n) for (int i = int(n); i >= 1; i--)
#define repc(i, n) for (int i = 0; i <= int(n); i++)
#define rrepc(i, n) for (int i = int(n); i >= 0; i--)
#define repi(i, a, b) for (int i = int(a); i < int(b); i++)
#define repic(i, a, b) for (int i = int(a); i <= int(b); i++)
#define all(a) (a).begin(), (a).end()
#define bit32(x) (1 << (x))
#define bit64(x) (1ll << (x))
#define sz(v) ((int) v.size())

using namespace std;

using i64 = long long;
using f80 = long double;
using vi32 = vector<int>;
using vi64 = vector<i64>;
using vf80 = vector<f80>;
using vstr = vector<string>;

void yes() { cout << "Yes" << endl; exit(0); }
void no() { cout << "No" << endl; exit(0); }
template <typename T> class pqasc : public priority_queue<T, vector<T>, greater<T>> {};
template <typename T> class pqdesc : public priority_queue<T, vector<T>, less<T>> {};
template <typename T> void amax(T &x, T y) { x = max(x, y); }
template <typename T> void amin(T &x, T y) { x = min(x, y); }
template <typename T> T exp(T x, i64 n, T e = 1) { T r = e; while (n > 0) { if (n & 1) r *= x; x *= x; n >>= 1; } return r; }
template <typename T> istream& operator>>(istream &is, vector<T> &v) { for (auto &x : v) is >> x; return is; }
template <typename T> ostream& operator<<(ostream &os, vector<T> &v) { rep(i, v.size()) { if (i) os << ' '; os << v[i]; } return os; }
void solve(); int main() { ios::sync_with_stdio(0); cin.tie(0); cout << fixed << setprecision(16); solve(); return 0; }

const int INF = 1001001001;
const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};

struct UnionFind {
  vector<int> data;
  UnionFind(int n) {
    data.assign(n, -1);
  }
  int root(int k) {
    if (data[k] < 0) return k;
    return data[k] = root(data[k]);
  }
  void unite(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y) return;
    if (data[x] > data[y]) swap(x, y);
    data[x] += data[y];
    data[y] = x;
  }
  bool same(int x, int y) {
    return root(x) == root(y);
  }
  int size(int k) {
    return -data[root(k)];
  }
};

template <int mod>
struct ModInt {
  int x;
  ModInt(): x(0) {}
  ModInt(long long a) { x = a % mod; if (x < 0) x += mod; }
  ModInt &operator+=(ModInt that) { x = (x + that.x) % mod; return *this; }
  ModInt &operator-=(ModInt that) { x = (x + mod - that.x) % mod; return *this; }
  ModInt &operator*=(ModInt that) { x = (long long) x * that.x % mod; return *this; }
  ModInt &operator/=(ModInt that) { return *this *= that.inverse(); }
  ModInt inverse() {
    int a = x, b = mod, u = 1, v = 0;
    while (b) { int t = a / b; a -= t * b; u -= t * v; swap(a, b); swap(u, v); }
    return ModInt(u);
  }
  #define op(o, p) ModInt operator o(ModInt that) { return ModInt(*this) p that; }
    op(+, +=) op(-, -=) op(*, *=) op(/, /=)
  #undef op
  friend ostream& operator<<(ostream &os, ModInt m) { return os << m.x; }
};

using mint = ModInt<1000000007>;

void solve() {
  int n;
  cin >> n;
  vi32 p(n);
  cin >> p;
  rep(i, n) if (p[i] != -1) p[i]--;
  int m = 0;
  rep(i, n) if (p[i] == -1) m += 1;
  vi32 a(m);
  int cycle = 0;
  {
    UnionFind uf(n);
    rep(i, n) if (p[i] != -1) {
      if (uf.same(i, p[i])) {
        cycle += 1;
      } else {
        uf.unite(i, p[i]);
      }
    }
    if (m == 0) {
      set<int> se;
      rep(i, n) {
        se.insert(uf.root(i));
      }
      cout << n - se.size() << endl;
      return;
    }
    int index = 0;
    rep(i, n) if (p[i] == -1) {
      a[index] = uf.size(i);
      index += 1;
    }
  }
  mint ans = 0;
  ans += (mint) n * exp((mint) n - 1, m);
  ans -= (mint) cycle * exp((mint) n - 1, m);
  rep(i, m) {
    ans -= (mint) (a[i] - 1) * exp((mint) n - 1, m - 1);
  }
  using vm = vector<mint>;
  vector<vm> dp(m + 1, vm(m + 1));
  dp[0][0] = 1;
  reps(i, m) repc(j, m) {
    dp[i][j] += dp[i - 1][j];
    if (j > 0) {
      dp[i][j] += dp[i - 1][j - 1] * a[i - 1];
    }
  }
  vm fac(m);
  fac[0] = 1;
  reps(i, m - 1) fac[i] = fac[i - 1] * i;
  repic(i, 2, m) {
    ans -= dp[m][i] * fac[i - 1] * exp((mint) n - 1, m - i);
  }
  cout << ans << endl;
}


Measured memory_bytes: 101048