#include <bits/stdc++.h>   using namespace std;   const double eps = 1e-6;   const int INF = 2e9;   const long long LNF = 9e18;   const int mod = 1e9 + 7;   const int maxn = 2e3 + 10;   int n, m, k;   bool val[maxn], pos[maxn];   long long C[maxn][maxn], A[maxn];   void init() {     scanf( %d , &n);     for (int i = 1; i <= n; i++) {       int x;       scanf( %d , &x);       if (x != -1) val[x] = pos[i] = 1;     }     k = m = 0;     for (int i = 1; i <= n; i++) {       if (!pos[i]) m++;       if (!pos[i] && !val[i]) k++;     }     A[0] = 1;     C[0][0] = 1;     for (int i = 1; i <= m; i++) {       A[i] = (1LL * i * A[i - 1]) % mod;       C[i][0] = 1;       for (int j = 1; j <= i; j++)         C[i][j] = (C[i - 1][j - 1] + C[i - 1][j]) % mod;     }   }   void solve() {     long long ans = A[m];     for (int i = 1; i <= k; i++) {       long long tmp = (1LL * C[k][i] * A[m - i]) % mod;       ans -= (i & 1) ? tmp : -tmp;       ans = (ans + mod) % mod;     }     cout << ans << endl;   }   int main() {     init();     solve();   }   