Iahub is so happy about inventing bubble sort graphs that he's staying all day long at the office and writing permutations. Iahubina is angry that she is no more important for Iahub. When Iahub goes away, Iahubina comes to his office and sabotage his research work.

The girl finds an important permutation for the research. The permutation contains n distinct integers a1, a2, ..., an (1 ≤ ai ≤ n). She replaces some of permutation elements with -1 value as a revenge. 

When Iahub finds out his important permutation is broken, he tries to recover it. The only thing he remembers about the permutation is it didn't have any fixed point. A fixed point for a permutation is an element ak which has value equal to k (ak = k). Your job is to proof to Iahub that trying to recover it is not a good idea. Output the number of permutations which could be originally Iahub's important permutation, modulo 1000000007 (109 + 7).

Input

The first line contains integer n (2 ≤ n ≤ 2000). On the second line, there are n integers, representing Iahub's important permutation after Iahubina replaces some values with -1. 

It's guaranteed that there are no fixed points in the given permutation. Also, the given sequence contains at least two numbers -1 and each positive number occurs in the sequence at most once. It's guaranteed that there is at least one suitable permutation.

Output

Output a single integer, the number of ways Iahub could recover his permutation, modulo 1000000007 (109 + 7).

Examples

Input

5
-1 -1 4 3 -1


Output

2

Note

For the first test example there are two permutations with no fixed points are [2, 5, 4, 3, 1] and [5, 1, 4, 3, 2]. Any other permutation would have at least one fixed point. 
import java.io.*;
import java.util.*;
/*
 * Heart beats fast
 * 		Colors and promises
 * 			How to be brave
 * 				How can I love when I am afraid...
 */
//read the question correctly (is y a vowel? what are the exact constraints?)
//look out for SPECIAL CASES (n=1?) and overflow (ll vs int?)
//always declare multidimensional arrays as [2][n] not [n][2]
//it can lead to upto 2-3x diff in runtime
//declare int/long tries with 16 array size due to object overhead :D
public class Main
{
	static long[][]subroutinedp;
	static long subroutinesolver(int n,int k)
	{
		if(k>n)
			return 0;
		if(subroutinedp[n][k]!=-1)
			return subroutinedp[n][k];
		long ans=0;
		if(k==0)
			ans=fact[n];
		else if(k==1)
			ans=((n-1)*Main.subroutinesolver(n-1, 0))%mod;
		else
		{
			ans=((n-k)*Main.subroutinesolver(n-1, k-1))%mod;
			ans+=((k-1)*Main.subroutinesolver(n-1, k-2))%mod;
		}
		return subroutinedp[n][k]=ans;
	}
	static long[]fact;
	public static void main(String[] args) throws Exception
	{
		fact=new long[2001];
		fact[0]=1;
		for(int i=1; i<=2000; i++)
			fact[i]=(fact[i-1]*i)%mod;
		subroutinedp=new long[2001][2001];
		for(int i=0; i<=2000; i++)
			Arrays.fill(subroutinedp[i], -1);
		int n=ni();
		int[]a=pnia(n);
		boolean[]non=new boolean[n+1],nmiss=new boolean[n+1];
		for(int i=1; i<=n; i++)
			if(a[i]==-1)
				non[i]=true;
			else
				nmiss[a[i]]=true;
		int te1=0,te2=0;
		for(int i=1; i<=n; i++)
			if(!nmiss[i])
				if(!non[i])
					te1++;
				else
					te2++;
		subroutinedp[0][0]=1;
		pr((Main.subroutinesolver(te1+te2,te2)%mod+mod)%mod);
		System.out.print(output);
	}
	///////////////////////////////////////////
	///////////////////////////////////////////
	///template from here
	static class pair
	{
		int a, b;
		pair(){}
		pair(int c,int d){a=c;b=d;}
	}
	static interface combiner
	{
		public int combine(int a, int b);
	}
	static final int mod=1000000007;
	static final double eps=1e-9;
	static final long inf=100000000000000000L;
	static Reader in=new Reader();
	static StringBuilder output=new StringBuilder();
	static Random rn=new Random();
	static void reverse(int[]a){for(int i=0; i<a.length/2; i++){a[i]^=a[a.length-i-1];a[a.length-i-1]^=a[i];a[i]^=a[a.length-i-1];}}
	static int log2n(long a)
	{
		int te=0;
		while(a>0)
		{
			a>>=1;
			++te;
		}
		return te;
	}
	static class vectorl implements Iterable<Long>
	{
		long a[];
		int size;
		vectorl(){a=new long[10];size=0;}
		vectorl(int n){a=new long[n];size=0;}
		public void add(long b){if(++size==a.length)a=Arrays.copyOf(a, 2*size);a[size-1]=b;}
		public void sort(){Arrays.sort(a, 0, size);}
		public void sort(int l, int r){Arrays.sort(a, l, r);}
		@Override
		public Iterator<Long> iterator() {
			Iterator<Long> hola=new Iterator<Long>()
					{
				int cur=0;
						@Override
						public boolean hasNext() {
							return cur<size;
						}
						@Override
						public Long next() {
							return a[cur++];
						}				
					};
			return hola;
		}
	}
	static class vector implements Iterable<Integer>
	{
		int a[],size;
		vector(){a=new int[10];size=0;}
		vector(int n){a=new int[n];size=0;}
		public void add(int b){if(++size==a.length)a=Arrays.copyOf(a, 2*size);a[size-1]=b;}
		public void sort(){Arrays.sort(a, 0, size);}
		public void sort(int l, int r){Arrays.sort(a, l, r);}
		@Override
		public Iterator<Integer> iterator() {
			Iterator<Integer> hola=new Iterator<Integer>()
					{
				int cur=0;
						@Override
						public boolean hasNext() {
							return cur<size;
						}
						@Override
						public Integer next() {
							return a[cur++];
						}				
					};
			return hola;
		}
	}
	//output functions////////////////
	static void pr(Object a){output.append(a+"\n");}
	static void pr(){output.append("\n");}
	static void p(Object a){output.append(a);}
	static void pra(int[]a){for(int i:a)output.append(i+" ");output.append("\n");}
	static void pra(long[]a){for(long i:a)output.append(i+" ");output.append("\n");}
	static void pra(String[]a){for(String i:a)output.append(i+" ");output.append("\n");}
	static void pra(double[]a){for(double i:a)output.append(i+" ");output.append("\n");}
	static void sop(Object a){System.out.println(a);}
	static void flush(){System.out.print(output);output=new StringBuilder();}
	//////////////////////////////////
	//input functions/////////////////
	static int ni(){return Integer.parseInt(in.next());}
	static long nl(){return Long.parseLong(in.next());}
	static String ns(){return in.next();}
	static double nd(){return Double.parseDouble(in.next());}
	static int[] nia(int n){int a[]=new int[n];for(int i=0; i<n; i++)a[i]=ni();return a;}
	static int[] pnia(int n){int a[]=new int[n+1];for(int i=1; i<=n; i++)a[i]=ni();return a;}
	static long[] nla(int n){long a[]=new long[n];for(int i=0; i<n; i++)a[i]=nl();return a;}
	static String[] nsa(int n){String a[]=new String[n];for(int i=0; i<n; i++)a[i]=ns();return a;}
	static double[] nda(int n){double a[]=new double[n];for(int i=0; i<n; i++)a[i]=nd();return a;}
	//////////////////////////////////
	//some utility functions
	static void exit(){System.out.print(output);System.exit(0);}
	static int min(int... a){int min=a[0];for(int i:a)min=Math.min(min, i);return min;}
	static int max(int... a){int max=a[0];for(int i:a)max=Math.max(max, i);return max;}	
	static int gcd(int... a){int gcd=a[0];for(int i:a)gcd=gcd(gcd, i);return gcd;}	
	static long min(long... a){long min=a[0];for(long i:a)min=Math.min(min, i);return min;}
	static long max(long... a){long max=a[0];for(long i:a)max=Math.max(max, i);return max;}	
	static long gcd(long... a){long gcd=a[0];for(long i:a)gcd=gcd(gcd, i);return gcd;}	
	static String pr(String a, long b){String c="";while(b>0){if(b%2==1)c=c.concat(a);a=a.concat(a);b>>=1;}return c;}
	static long powm(long a, long b, long m){long an=1;long c=a;while(b>0){if(b%2==1)an=(an*c)%m;c=(c*c)%m;b>>=1;}return an;}
	static int gcd(int a, int b){if(b==0)return a;return gcd(b, a%b);}
	static long gcd(long a, long b){if(b==0)return a;return gcd(b, a%b);}
	static class Reader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;
        public Reader() {
            reader = new BufferedReader(new InputStreamReader(System.in), 32768);
            tokenizer = null;
        }
        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }
    }
}