import tensorflow as tf


    # casting to float on GPU ensures lower data transfer times.
    if lander:
      obs_t_float = self.obs_t_ph
      obs_tp1_float = self.obs_tp1_ph
    else:
      obs_t_float   = tf.cast(self.obs_t_ph,   tf.float32) / 255.0
      obs_tp1_float = tf.cast(self.obs_tp1_ph, tf.float32) / 255.0

    # Here, you should fill in your own code to compute the Bellman error. This requires
    # evaluating the current and next Q-values and constructing the corresponding error.
    # TensorFlow will differentiate this error for you, you just need to pass it to the
    # optimizer. See assignment text for details.
    # Your code should produce one scalar-valued tensor: total_error
