1 //#####################
 2 //Author:fraud
 3 //Blog: http://www.cnblogs.com/fraud/
 4 //#####################
 5 #include <iostream>
 6 #include <sstream>
 7 #include <ios>
 8 #include <iomanip>
 9 #include <functional>
10 #include <algorithm>
11 #include <vector>
12 #include <string>
13 #include <list>
14 #include <queue>
15 #include <deque>
16 #include <stack>
17 #include <set>
18 #include <map>
19 #include <cstdio>
20 #include <cstdlib>
21 #include <cmath>
22 #include <cstring>
23 #include <climits>
24 #include <cctype>
25 using namespace std;
26 #define XINF INT_MAX
27 #define INF 0x3FFFFFFF
28 #define MP(X,Y) make_pair(X,Y)
29 #define PB(X) push_back(X)
30 #define REP(X,N) for(int X=0;X<N;X++)
31 #define REP2(X,L,R) for(int X=L;X<=R;X++)
32 #define DEP(X,R,L) for(int X=R;X>=L;X--)
33 #define CLR(A,X) memset(A,X,sizeof(A))
34 #define IT iterator
35 typedef long long ll;
36 typedef pair<int,int> PII;
37 typedef vector<PII> VII;
38 typedef vector<int> VI;
39 const ll MOD =1000000007 ;
40 int a[10010];
41 ll b[2010];
42 bool vis[2010];
43 ll dp[2010];
44 int main()
45 {
46     ios::sync_with_stdio(false);
47     int n;
48     cin>>n;
49     b[0]=1;
50     for(int i=1;i<=n;i++)cin>>a[i];
51     for(ll i=1;i<2010;i++)b[i]=(b[i-1]*i)%MOD;
52     int tx=0,ty=0;
53     for(int i=1;i<=n;i++){
54         if(a[i]!=-1)vis[a[i]]=1;
55     }
56     for(int i=1;i<=n;i++){
57         if(a[i]==-1){
58             if(vis[i])tx++;
59             else ty++;
60         }
61     }
62     dp[0]=1;
63     dp[1]=tx*dp[0]%MOD;
64     for(int i=2;i<=ty;i++){
65         dp[i]=(tx*dp[i-1]%MOD+(i-1)*dp[i-1]%MOD+(i-1)*dp[i-2]%MOD)%MOD;
66     }
67     cout<<dp[ty]*b[tx]%MOD<<endl;
68
69
70
71
72
73     return 0;
74 }