# Write your Python code here
total = 0
count = 0

while True:
    grade = float(input("Enter a grade (or -1 to finish): "))

    if grade == -1:
        break

    total += grade
    count += 1

if count > 0:
    average = total / count
    print(f"Average grade: {average:.2f}")

    if marks >= 90:
        letter = "A"
    elif marks >= 80:
        letter = "B"
    elif marks >= 70:
        letter = "C"
    elif marks >= 60:
        letter = "D"
    else:
        letter = "F"

    print(f"Letter grade: {letter}")
else:
    print("No grades were entered.")