input: Write a python function to define a neural network using TensorFlow with the given structure.
  assert define_neural_network(100, 10).layers[0].units == 50
  assert define_neural_network(100, 10).layers[1].units == 40
  assert define_neural_network(100, 10).layers[2].units == 30
  assert define_neural_network(100, 10).layers[3].units == 10
output: def define_neural_network(input_dim, output_dim):
      import tensorflow as tf
      from tensorflow.keras.models import Sequential
      from tensorflow.keras.layers import Dense
      
      model = Sequential([
          Dense(50, input_dim=input_dim, activation='relu'),
          Dense(40, activation='relu'),
          Dense(30, activation='relu'),
          Dense(output_dim, activation='softmax')
      ])
      
      return model