#include<bits/stdc++.h>
#define file(x) freopen(x".in","r",stdin),freopen(x".out","w",stdout)
#define ll long long
using namespace std;
const int N=110,p=998244353;
int n,m,a[N],f[N][N][N],C[N][N];
void add(int &x,int y){x+=y;if(x>=p)x-=p;}
signed main(){
//  file("graph");
    scanf("%d",&n);
    for(int i=1,x;i<=n;i++){
        scanf("%d",&x);
        if(x!=-1)a[x]++;
        else ++m;
    }
    for(int i=0;i<=n;i++){
        C[i][0]=1;
        for(int j=1;j<=i;j++)C[i][j]=(C[i-1][j-1]+C[i-1][j])%p;
    }
    f[n][m][0]=1;
    for(int i=n-1;i>=1;i--){
        for(int j=0;j<=m;j++){
            for(int k=0;k<=n;k++)if(f[i+1][j][k])
                for(int l=0;l<=j;l++){
                    if(a[i]+l+k>=2){
                        add(f[i][j-l][a[i]+l+k>>1],1ll*f[i+1][j][k]*C[j][l]%p);
                    }else if(a[i]+l+k==1);
                    else add(f[i][j-l][0],1ll*f[i+1][j][k]*C[j][l]%p);
                }
        }
    }
    int ans=0;
    for(int k=1;k<=n;k++)add(ans,f[1][0][k]);
    printf("%d\n",ans);
}