def getGrade():
  # step 1
  result = 'X' # the default value 

  # step 2
  while True:  
    result = input("What is your grade? [A, B, C, D, F, or X to quit]: ")[0] 

    if result == 'A' or result == 'B' or result == 'C': break
    if result == 'D' or result == 'F': break
    if result == 'X' or result == 'x': break

    print(result, "is not a valid grade. Try again...") 

  # step 3
  return result
# getGrade ends here

# main program starts here  
while True: 
  grade = getGrade() 
  if grade == 'X' or grade == 'x': break; 
  if grade == 'A' or grade == 'B' or grade == 'C':
    print("You pass") 
  
print("Thanks for checking your grades!")