# 파이썬은 ++ 안됨!
# f-string 쓰는법 공부하기
a1 = int(input())
a2 = int(input())
a3 = int(input())
a4 = int(input())
a5 = int(input())
# a1, a2, a3, a4, a5 = x.split("")
avg = (a1 + a2 + a3 + a4 + a5) / 5.00
min = a1
arr = [a1, a2, a3, a4, a5]
count = 0

for i in range(len(arr)):
    if (arr[i] < 0 or arr[i] > 100):
        print("Invalid")
        exit()
for i in range(len(arr)):
    if (arr[i] < 40):
        count += 1
if (count >= 3):
    print("F")
else:
    if (avg >= 90):
        print("A+")
    elif (avg >= 75):
            print("A")
    elif (avg >= 65):
        print("B+")
    elif (avg >= 50):
        print("B")
    else:
        print("F")
if (min > a2):
    min = a2
if (min > a3):
    min = a3
if (min > a4):
    min = a4
if (min > a5):
    min = a5
print(f"avg={avg:.2f} min={min}")
# print("avg=" + str(avg) + " min=" + str(min))