Bitmask

#include <bits/stdc++.h>
#define ll long long
#define pb push_back

using namespace std;

int main() {
    int n, k;
    cin >> n >> k;
    int w[n];
    for (int i = 0; i < n; ++i) {
        cin >> w[i];
    }

    if (n > 20) {
        cout << "1";
        return 0;
    }

    int ans = 0;

    for (int mask = 0; mask < (1 << n); ++mask) {
        int cur = 0;
        set<int> b;

        for (int i = n - 1; i >= 0; --i) {
            if ((mask & (1 << i)) != 0) {
                cur++;
                b.insert(w[i]);
            }
        }

        if (cur == k && b.size() == k) {
            ans++;
        }
    }

    cout << ans;

    return 0;
}