#include<bits/stdc++.h>
#define ll long long
#define db double

using namespace std;

const int Mod = 998244353;

int f[105][105][105];

void add(int&x, int y) {
    x += y; 
    x >= Mod ? x -= Mod : 0;
}

int Fpow(int x, int k) {
    int ans = 1; 
    for (; k; k >>= 1, x = 1ll * x * x % Mod) {
        if (k & 1) {
            ans = 1ll * ans * x % Mod;
        }
    }
    return ans;
}

void solve() {
    int n;
    cin >> n;
    vector<int> a(n, 0);
    int m = 0;
    vector<int> Inv(n + 1, 0), Fac(n + 1, 0);
    Fac[0] = 1;
    for (int i = 1; i <= n; i++) {
        Fac[i] = 1ll * Fac[i - 1] * i % Mod;
    }
    Inv[n] = Fpow(Fac[n], Mod - 2);
    for (int i = n; i >= 1; i--) {
        Inv[i - 1] = 1ll * Inv[i] * i % Mod;
    }
    for (int i = 0; i < n; i++) {
        int x;
        cin >> x;
        if (x == -1) m++;
        else a[x]++;
    }
    f[n][0][0] = 1;
    for (int t = n - 1; t >= 1; t--) {
        for (int num = 0; num <= n; num++) {
            for (int p = 0; num + p + a[t] <= n; p++) {
                for (int l = 0; (l + p + a[t] >> 1) <= n; l++) {
                    if (l + p + a[t] != 1) {
                        add(f[t][(l + p + a[t] >> 1)][num + p + a[t]], 1ll * f[t + 1][l][num] * Inv[p] % Mod);
                    }
                }
            }
        }
    }
    int ans = 0;
    for(int t = 1; t <= n; t++) {
        add(ans, f[1][t][n]);
    }
    cout << 1ll * ans * Fac[m] % Mod << '\n';
}

int main() {
    // freopen(".in", "r", stdin);
    // freopen(".out", "w", stdout);
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);
    int t = 1;
    // cin >> t;
    while (t--) solve();
    return 0;
}