#include<bits/stdc++.h>

using namespace std;

#define Fopen freopen("_in.txt","r",stdin); freopen("_out.txt","w",stdout);

#define LL long long

#define ULL unsigned LL

#define fi first

#define se second

#define pb push_back

#define lson l,m,rt<<1

#define rson m+1,r,rt<<1|1

#define lch(x) tr[x].son[0]

#define rch(x) tr[x].son[1]

#define max3(a,b,c) max(a,max(b,c))

#define min3(a,b,c) min(a,min(b,c))

typedef pair<int,int> pll;

const int inf = 0x3f3f3f3f;

const int _inf = 0xc0c0c0c0;

const LL INF = 0x3f3f3f3f3f3f3f3f;

const LL _INF = 0xc0c0c0c0c0c0c0c0;

const LL mod =  (int)1e9+;

const int N = 1e5 + ;

LL dp[N];

int fvis[N];

int vis[N];

int cnt1, cnt2;

int main(){

    int n, t;

    scanf("%d", &n);

    for(int i = ; i <= n; ++i){

        scanf("%d", &t);

        if(t == -) continue;

        vis[t] = ;

        fvis[i] = ;

    }

    for(int i = ; i <= n; ++i){

        if(vis[i] + fvis[i] == ) ;

        else if(!vis[i] && fvis[i]) cnt1++;

        else if(vis[i] + fvis[i] == ) cnt2++;

    }

    dp[] = ;

//    cout << cnt1 << " " << cnt2 << endl;

    for(int i = ; i <= cnt1; ++i) dp[] = dp[] * i % mod;

    for(int i = ; i <= cnt2; ++i){

        dp[i] = cnt1 * dp[i-] % mod + (i-) * dp[i-] % mod;

        if(i >= ) dp[i] += (i-) * dp[i-] % mod;

        dp[i] %= mod;

//        cout << dp[i] << endl;

    }

    cout << dp[cnt2] << endl;

    return ;

}