#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
#include <cmath>
#include <map>
#include <string>
#include <fstream>
#include <queue>

using namespace std;

ifstream in("input.txt");
ofstream out("output.txt");

typedef long long ll;

vector<int> a;
vector<bool> flag;
vector<int> ans;
int res = 0;
int n;

void rec(int v) {
    if (v == n) {
        bool found = true;
        for (int i = 0; i < n; ++i) {
            if (ans[i] < i + 1)
                found = false;
        }
        if (found)
            ++res;
        return;
    }
    for (int i = 0; i < n; ++i) {
        if (!flag[i]) {
            flag[i] = true;
            ans.push_back(a[i]);
            rec(v + 1);
            flag[i] = false;
            ans.pop_back();
        }
    }
}


int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    in >> n;
    a.resize(n);
    flag.resize(n);
    for (int i = 0; i < n; ++i) {
        in >> a[i];
    }
    rec(0);
    out << res << endl;
}