from keras import backend as K
if K.image_data_format() == 'channels_first' :
   input_shape = (1, 28, 28)
   X_train = X_train.reshape(X_train.shape[0], 1, 28, 28)
   X_test = X_test.reshape(X_test.shape[0], 1, 28, 28)
else:
   input_shape = (28, 28, 1)
   X_train = X_train.reshape(X_train.shape[0], 28, 28, 1)
   X_test = X_test.reshape(X_test.shape[0], 28, 28, 1)

model.add(Input((28 * 28,)))

images_array = images_array.reshape(-1, 28 * 28)