struct shoes_compare {
    bool operator()(int left, int right) {
        if (left - right >= 3 || right == left) {
            return true;
        }
        else {
            return false;
        }
    }
};


void task2() {
    int customer_size, n;
    cin >> customer_size >> n;
    vector <int> shoes;
    for (int i = 0; i < n; i++) {
        int current_shoes;
        cin >> current_shoes;
        shoes.push_back(current_shoes);
    }
    sort(shoes.begin(), shoes.end());
    auto it = lower_bound(shoes.begin(), shoes.end(), customer_size, shoes_compare());
    int valid_shoes = 0;
    if (it != shoes.end()) {
        valid_shoes = 1;
    }
    else {
        cout << valid_shoes;
        return;
    }
    auto last_valid_shoes = it;
    for (it = it + 1; it != shoes.end(); it++) {
        if ((*it - *(last_valid_shoes)) >= 3) {
            valid_shoes++;
            last_valid_shoes = it;
        }
    }
    cout << valid_shoes;
}