class Student:

  def __init__(self,name):
    self._name = name
    self._courses = []

  def add_course(self,cno,credits,grade):
    self._courses.append((cno,credits,grade))
  
  def gpa(self):
    grade_points = {'A': 4, 'B': 3, 'C': 2, 'D': 1, 'F': 0}
    points = 0
    total_credits = 0
    for (cn,cr,g) in self._courses:
      points += cr*grade_points[g]
      total_credits += cr
    if (total_credits > 0):
      return round(points/total_credits,2)
    else:
      return None