input: Write a python function to calculate the fitness of a given state in a genetic algorithm.
  assert GeneticAlgo((1, 2, 3, 4), (4, 3, 2, 1), 2, 10)._calculate_fitness((4, 3, 2, 1)) == 4
  assert GeneticAlgo((1, 2, 3, 4), (4, 3, 2, 1), 2, 10)._calculate_fitness((1, 2, 3, 4)) == 0
  assert GeneticAlgo((1, 2, 3, 4), (4, 3, 2, 1), 2, 10)._calculate_fitness((4, 2, 3, 1)) == 2
output: class GeneticAlgo:
      def __init__(self, initial_state, target_state, mutation_rate, population_size):
          self.initial_state = initial_state
          self.target_state = target_state
          self.mutation_rate = mutation_rate
          self.population_size = population_size
  
      def _calculate_fitness(self, state):
          fitness = 0
          for a, b in zip(state, self.target_state):
              if a != b:
                  fitness += 1
          return fitness