#include <bits/stdc++.h>
#include <math.h>
#include <float.h>
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rrep(i, n) for(int i = 0; i <= (n); i++)
using namespace std;
typedef long long ll;
 
const ll INF = 1LL<<62;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
// 多次元 vector 生成
template<class T>
vector<T> make_vec(size_t a){
    return vector<T>(a);
}
template<class T, class... Ts>
auto make_vec(size_t a, Ts... ts){
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

//const int MOD = 1000000007;
const int MOD = 998244353;
struct mint {
    ll x;
    mint(ll x=0):x((x%MOD+MOD)%MOD){}
    mint operator-() const { return mint(-x);}
    mint& operator+=(const mint a) {
        if ((x += a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator-=(const mint a) {
        if ((x += MOD-a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator*=(const mint a) {
        (x *= a.x) %= MOD;
        return *this;
    }
    bool operator==(const mint a){
        return (x == a.x);
    }
    mint operator+(const mint a) const {
        mint res(*this);
        return res+=a;
    }
    mint operator-(const mint a) const {
        mint res(*this);
        return res-=a;
    }
    mint operator*(const mint a) const {
        mint res(*this);
        return res*=a;
    }
    mint pow(ll t) const {
        if (!t) return 1;
        mint a = pow(t>>1);
        a *= a;
        if (t&1) a *= *this;
        return a;
    }
    // for prime MOD
    mint inv() const {
        return pow(MOD-2);
    }
    mint& operator/=(const mint a) {
        return (*this) *= a.inv();
    }
    mint operator/(const mint a) const {
        mint res(*this);
        return res/=a;
    }
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}
// 2項係数を求める
struct combination {
    vector<mint> fact, ifact;
    combination(int n):fact(n+1),ifact(n+1) {
        assert(n < MOD);
        fact[0] = 1;
        for (int i = 1; i <= n; ++i) fact[i] = fact[i-1]*i;
        ifact[n] = fact[n].inv();
        for (int i = n; i >= 1; --i) ifact[i-1] = ifact[i]*i;
    }
    mint operator()(int n, int k) {
        if (k < 0 || k > n) return 0;
        return fact[n]*ifact[k]*ifact[n-k];
    }
};

//nが非常に大きく、aが小さい場合
mint choose(int n, int a){
    mint x = 1, y = 1;
    rep(i,a){
        x *= n-i;
        y *= i+1;
    }
    return x/y;
}

int main(){
    int N; cin >> N;
    vector<int> D(N);
    rep(i,N) cin >> D[i];

    if (D[0] != 0){
        cout << 0 << endl;
        return 0;
    }
    else{
        for (int i = 1; i < N; i++){
            if (D[i] == 0){
                cout << 0 << endl;
                return 0;
            }
        }
    }
    /*
    sort(D.begin(), D.end());

    rep(i,N){
        if (i == 0) continue;
        if (abs(D[i-1]-D[i]) > 1){
            cout << 0 << endl;

        }
    }
    */
    vector<int> counter(N,0);
    rep(i,N){
        counter[D[i]]++;
    }
    

    mint ans = 1;
    for (int i = 2; i < N; i++){
        rep(j,counter[i]){
            ans *= counter[i-1];
        }
    }

    cout << ans << endl; 

}

Measured memory_bytes: 4052