_%E1%84%8A%E1%85%A5%E1%86%B7%E1%84%82%E1%85%A6%E1%84%8B%E1%85%B5%E1%86%AF.png)

input_data = int(input())
if input_data >= 90:
print('A')
elif input_data < 90 and input_data >= 80:
print('B')
elif input_data < 80 and input_data >= 70:
print('C')
elif input_data < 70 and input_data >= 60:
print('D')
else:
print('F')