// P = 1*2 + 2*3*4 + 3*4*5*6 + ... + N*(N+1)*...*2N.
    private int huita(int N) {
        int result = 0;
        for (int i = 1; i <= N; i++){
            int part = 1;
            for (int j = i; j <= 2*i; j++) {
                part = part * j;
            }
            result = result + part;
        }
        return result;
    }