#include <bits/stdc++.h>
using namespace std;
double EPS = 1e-9;
int INF = 2000000000;
long long INFF = 8000000000000000000LL;
double PI = acos(-1);
int dirx[8] = {-1, 0, 0, 1, -1, -1, 1, 1};
int diry[8] = {0, 1, -1, 0, -1, 1, -1, 1};
inline string IntToString(int a) {
  char x[100];
  sprintf(x, "%d", a);
  string s = x;
  return s;
}
inline int StringToInt(string a) {
  char x[100];
  int res;
  strcpy(x, a.c_str());
  sscanf(x, "%d", &res);
  return res;
}
inline string GetString(void) {
  char x[1000005];
  scanf("%s", x);
  string s = x;
  return s;
}
inline string uppercase(string s) {
  int n = (int)s.size();
  for (int(i) = (0); (i) < (n); ++(i))
    if (s[i] >= 'a' && s[i] <= 'z') s[i] = s[i] - 'a' + 'A';
  return s;
}
inline string lowercase(string s) {
  int n = (int)s.size();
  for (int(i) = (0); (i) < (n); ++(i))
    if (s[i] >= 'A' && s[i] <= 'Z') s[i] = s[i] - 'A' + 'a';
  return s;
}
inline void OPEN(string s) {
  freopen((s + ".in").c_str(), "r", stdin);
  freopen((s + ".out").c_str(), "w", stdout);
}
vector<int> angka;
int n, data[2005];
long long dewa[2005];
long long MOD = 1000000007;
bool sudah[2005];
int to[2005];
int conn;
int pisah;
long long risan;
long long C[2005][2005];
int main() {
  scanf("%d", &n);
  for (int(i) = (1); (i) <= (n); ++(i)) {
    scanf("%d", &data[i]);
    to[i] = data[i];
  }
  for (int(i) = (0); (i) <= (n); ++(i)) {
    C[i][0] = C[i][i] = 1;
    for (int(j) = (1); (j) <= (i - 1); ++(j))
      C[i][j] = (C[i - 1][j - 1] + C[i - 1][j]) % MOD;
  }
  dewa[1] = 0;
  dewa[2] = 1;
  dewa[0] = 1;
  for (int(i) = (3); (i) <= (n); ++(i)) {
    dewa[i] = (i - 1) * (dewa[i - 1] + dewa[i - 2]);
    dewa[i] %= MOD;
  }
  memset(sudah, 0, sizeof(sudah));
  for (int(i) = (1); (i) <= (n); ++(i)) {
    if (to[i] == -1) continue;
    int x = i;
    while (x != -1) {
      if (sudah[x]) goto hell;
      sudah[x] = 1;
      x = to[x];
      if (x == i) goto hell;
    }
    ++conn;
  hell:;
  }
  for (int(i) = (1); (i) <= (n); ++(i))
    if (!sudah[i]) ++pisah;
  for (int(i) = (0); (i) <= (conn); ++(i)) {
    long long pos = (C[conn][i] * dewa[i + pisah]) % MOD;
    risan = (risan + pos) % MOD;
  }
  printf("%d\n", (int)risan);
  return 0;
}
