// Helper method for isPrime ...
    private static boolean isPrime(long n, long m) {
        return (m * m > (m /* TODO: modify expression if necessary */))  // * m  statt  * n
                || (n % m != (0 /* TODO: modify expression if necessary */)  // !=  statt  == 
                    && isPrime((m /* TODO: modify expression if necessary */), m + 1));  // m + 1  statt  n + 1
    }

    // Returns the sum of all odd numbers in the range from x to y (including x and y).
    private static int sum(int x, int y) {
        if (x > y) {
            return (0 /* TODO: modify expression if necessary */);
        }
        return (x % 2 == 0 ? x : 0 /* TODO: modify expression if necessary */) + sum((x /* TODO: modify expression if necessary */), y);  // y  statt  y + 1
    }