# Exercise 4.7

def computescore(scr):
    if scr >= 0.9 :
        print 'Grade: A'
    elif scr >= 0.8 :
        print 'Grade: B'
    elif scr >= 0.7 :
        print 'Grade: C'
    elif scr >= 0.6 :
        print 'Grade: D'
    else:
        print 'Grade: F'

try:
    scr = float(raw_input('Please enter a number between 0.0 and 1.0. \n'))
    if (scr > 1.0 or scr < 0.0):
        print 'Please enter a valid number.'
        quit()
except:
    print 'Please enter a number.'
    quit()

grade = computescore(scr)

print grade