score = int(input("Please enter the student's score. "))

if score >= 90:
    print("This student's score of " + str(score) + " is an A.")
else:
    if score >= 80:
        print("This student's score of " + str(score) + " is a B.")
    else:
        if score >= 70:
            print("This student's score of " + str(score) + " is a C.")
        else:
            if score >= 60:
                print("This student's score of " + str(score) + " is a D.")
            else:
                print("This student's score of " + str(score) + " is a F.")