#include<bits/stdc++.h>
#define rep(i,a,b) for (register int i=(a);i<=(b);i++)
#define drep(i,a,b) for (register int i=(a);i>=(b);i--)
typedef long long ll;
using namespace std;

const int mod=998244353;
inline int qmo(int x){return x+((x>>31)&mod);}
inline int add(int x,int y){return qmo(x+y-mod);}
inline int sub(int x,int y){return qmo(x-y);}
inline void inc(int &x,int y){x=add(x,y);}
inline void dec(int &x,int y){x=sub(x,y);}
inline int quick_pow(int x,int k){int res=1; for (;k;k>>=1,x=1ll*x*x%mod) if (k&1) res=1ll*res*x%mod; return res;}

const int N=200010;

int fac[N],_fac[N],inv[N];
inline void init(int size)
{
	*fac=*_fac=*inv=1;
	rep(i,1,size) fac[i]=1ll*fac[i-1]*i%mod;
	_fac[size]=quick_pow(fac[size],mod-2);
	drep(i,size-1,1) _fac[i]=1ll*_fac[i+1]*(i+1)%mod;
	rep(i,1,size) inv[i]=1ll*fac[i-1]*_fac[i]%mod;
}
inline int C(int n,int m){return n>=m&&m>=0?1ll*fac[n]*_fac[m]%mod*_fac[n-m]%mod:0;}

int main()
{
	ios::sync_with_stdio(false); cin.tie(0);
	init(N-10);
	
	int n; cin>>n;
	vector<int> cnt(n),a(n);
	int m=0;
	rep(i,0,n-1) cin>>a[i];
	rep(i,0,n-1) (a[i]!=-1)?cnt[a[i]]++:m++;
	
	vector<vector<int> > dp(m+1,vector<int>(n+1));
	dp[0][0]=1;
	
	drep(i,n-1,1)
	{
		vector<vector<int> > ndp(m+1,vector<int>(n+1));
		rep(j,0,m) rep(k,0,n) if (dp[j][k]) rep(t,0,m-j)
		{
			int v=cnt[i]+k+t;
			if (v==1) continue;
			inc(ndp[j+t][v/2],1ll*dp[j][k]*C(m-j,t)%mod);
		}
		swap(ndp,dp);
	}
	int ans=0;
	rep(i,0,n) inc(ans,dp[m][i]);
	cout<<ans<<endl;
	
	return 0;
}