input: Write a python function to calculate grade based on the given score.
  assert calculate_grade(90) == "A"
  assert calculate_grade(85) == "B"
  assert calculate_grade(75) == "C"
  assert calculate_grade(65) == "D"
  assert calculate_grade(50) == "F"
output: def calculate_grade(score):
      if score >= 90:
          return "A"
      elif score >= 80:
          return "B"
      elif score >= 70:
          return "C"
      elif score >= 60:
          return "D"
      else:
          return "F"