#include <bits/stdc++.h>
#pragma comment(linker, "/stack:20000000")
using namespace std;
template <typename T1, typename T2, typename T3>
struct triple {
  T1 a;
  T2 b;
  T3 c;
  triple(){};
  triple(T1 _a, T2 _b, T3 _c) : a(_a), b(_b), c(_c) {}
};
template <typename T1, typename T2, typename T3>
bool operator<(const triple<T1, T2, T3> &t1, const triple<T1, T2, T3> &t2) {
  if (t1.a != t2.a)
    return t1.a < t2.a;
  else if (t1.b != t2.b)
    return t1.b < t2.b;
  else
    return t1.c < t2.c;
}
template <typename T1, typename T2, typename T3>
ostream &operator<<(std::ostream &os, const triple<T1, T2, T3> &tr) {
  os << "{" << tr.a << "," << tr.b << "," << tr.c << "}";
  return os;
}
template <typename T1, typename T2>
inline std::ostream &operator<<(std::ostream &os, const std::pair<T1, T2> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <typename T>
inline std::ostream &operator<<(std::ostream &os, const std::vector<T> &v) {
  bool first = true;
  os << "[";
  for (unsigned int i = 0; i < v.size(); i++) {
    if (!first) os << ", ";
    os << v[i];
    first = false;
  }
  return os << "]";
}
template <typename T>
inline std::ostream &operator<<(std::ostream &os, const std::set<T> &v) {
  bool first = true;
  os << "[";
  for (typename std::set<T>::const_iterator ii = v.begin(); ii != v.end();
       ++ii) {
    if (!first) os << ", ";
    os << *ii;
    first = false;
  }
  return os << "]";
}
template <typename T1, typename T2>
inline std::ostream &operator<<(std::ostream &os, const std::map<T1, T2> &v) {
  bool first = true;
  os << "[";
  for (typename std::map<T1, T2>::const_iterator ii = v.begin(); ii != v.end();
       ++ii) {
    if (!first) os << ", ";
    os << *ii;
    first = false;
  }
  return os << "]";
}
template <typename T, typename T2>
void printarray(T a[], T2 sz, T2 beg = 0) {
  for (T2 i = beg; i < sz; i++) cout << a[i] << " ";
  cout << endl;
}
inline long long binpow(long long x, long long n) {
  long long res = 1;
  while (n) {
    if (n & 1) res *= x;
    x *= x;
    n >>= 1;
  }
  return res;
}
inline long long mulmod(long long x, long long n, long long _mod) {
  long long res = 0;
  while (n) {
    if (n & 1) res = (res + x) % _mod;
    x = (x << 1) % _mod;
    n >>= 1;
  }
  return res;
}
inline long long powmod(long long x, long long n, long long _mod) {
  long long res = 1;
  while (n) {
    if (n & 1) res = (res * x) % _mod;
    x = (x * x) % _mod;
    n >>= 1;
  }
  return res;
}
inline long long gcd(long long a, long long b) {
  long long t;
  while (b) {
    a = a % b;
    t = a;
    a = b;
    b = t;
  }
  return a;
}
inline int gcd(int a, int b) {
  int t;
  while (b) {
    a = a % b;
    t = a;
    a = b;
    b = t;
  }
  return a;
}
inline long long lcm(int a, int b) { return a / gcd(a, b) * (long long)b; }
inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
inline long long gcd(long long a, long long b, long long c) {
  return gcd(gcd(a, b), c);
}
inline int gcd(int a, int b, int c) { return gcd(gcd(a, b), c); }
inline long long lcm(long long a, long long b, long long c) {
  return lcm(lcm(a, b), c);
}
inline long long lcm(int a, int b, int c) {
  return lcm(lcm(a, b), (long long)c);
}
inline long long max(long long a, long long b) { return (a > b) ? a : b; }
inline int max(int a, int b) { return (a > b) ? a : b; }
inline double max(double a, double b) { return (a > b) ? a : b; }
inline long long max(long long a, long long b, long long c) {
  return max(a, max(b, c));
}
inline int max(int a, int b, int c) { return max(a, max(b, c)); }
inline double max(double a, double b, double c) { return max(a, max(b, c)); }
inline long long min(long long a, long long b) { return (a < b) ? a : b; }
inline int min(int a, int b) { return (a < b) ? a : b; }
inline double min(double a, double b) { return (a < b) ? a : b; }
inline long long min(long long a, long long b, long long c) {
  return min(a, min(b, c));
}
inline int min(int a, int b, int c) { return min(a, min(b, c)); }
inline double min(double a, double b, double c) { return min(a, min(b, c)); }
inline int fastMax(int x, int y) {
  return (((y - x) >> (32 - 1)) & (x ^ y)) ^ y;
}
inline int fastMin(int x, int y) {
  return (((y - x) >> (32 - 1)) & (x ^ y)) ^ x;
}
inline void getar(int *a, int n) {
  for (int ii = 0; ii < n; ii++) {
    scanf("%d", a + ii);
  }
}
inline void getar(long long *a, int n) {
  for (int ii = 0; ii < n; ii++) {
    scanf("%I64", a + ii);
  }
}
int a[100010];
long long fact[3000];
int pos[100010];
long long c[2001][2001];
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i <= n; i++) {
    c[i][0] = 0;
  }
  for (int i = 0; i <= n; i++) {
    c[0][i] = 1;
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      c[i][j] = (c[i - 1][j - 1] + c[i][j - 1]) % 1000000007;
    }
  }
  getar(a, n);
  int k = 0;
  int t = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] != -1) {
      pos[a[i]] = i + 1;
    } else {
      ++t;
    }
  }
  for (int i = 0; i < n; i++) {
    if (pos[i + 1] == 0 && a[i] == -1) {
      ++k;
    }
  }
  fact[0] = 1;
  for (int i = 1; i <= n; i++) {
    fact[i] = (fact[i - 1] * 1ll * i) % 1000000007;
  }
  long long cur = 0;
  long long ans = 0;
  for (int i = 0; i <= k; i++) {
    if (i & 1) {
      ans = (ans + 1000000007 - ((fact[t - i] * 1ll * c[i][k]) % 1000000007)) %
            1000000007;
    } else {
      ans = (ans + ((fact[t - i] * 1ll * c[i][k]) % 1000000007)) % 1000000007;
    }
  }
  cout << ans << endl;
  return 0;
}
