double random_position(void) {
    /* Not perfectly uniformly distributed but sufficient:
       return a random floating point value in range [-1.0; +1.0]. */
    return ((double) rand() / RAND_MAX) * 2.0 - 1.0;
}

int is_in_circle(double x, double y) {
    /* Pythagoras was here: x^2 + y^2 = r^2 */
    return x * x + y * y <= 1.0;
}

double monte_carlo_pi(int iterations) {
    int in_square_count = 0;
    int in_circle_count = 0;

    while (iterations-- > 0) {
        double x = random_position();
        double y = random_position();

        if (is_in_circle(x, y)) {
            ++in_circle_count;
        } else {
            ++in_square_count;
        }
    }

    if (in_square_count > 0) {
        return 4.0 * in_circle_count / in_square_count;
    }

    /* Not found. */
    return -1.0;
}