Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = [int(i) for i in input().solit()]

    if m == f == r == -1:
        break

    if m == -1 or f == -1:
        print("F")

    if m + f >= 80:
        print("A")

    if 65 <= m + f < 80:
        print("B")

    if 50 <= m + f < 65:
        print("C")

    if 30 <= m + f < 50:
        print("D")

    elif r >= 50:
        print("C")

    else:
        print("F")

Fixed submission:
while True:
    m, f, r = [int(i) for i in input().split()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r >= 50:
            print("C")
        else:
            print("D")

    elif m + f < 30:
        print("F")

    elif m + f < 30:
        print("F")

Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = [int(i) for i in input().solit()]

    if m == f == r == -1:
        break

    if m == -1 or f == -1:
        print("F")

    if m + f >= 80:
        print("A")

    if 65 <= m + f < 80:
        print("B")

    if 50 <= m + f < 65:
        print("C")

    if 30 <= m + f < 50:
        print("D")

    elif r >= 50:
        print("C")

    if m + f < 30:
        print("F")

Fixed submission:
while True:
    m, f, r = [int(i) for i in input().split()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r >= 50:
            print("C")
        else:
            print("D")

    elif m + f < 30:
        print("F")

    elif m + f < 30:
        print("F")

Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = [int(i) for i in input().solit()]

    if m == f == r == -1:
        break

    if m == -1 or f == -1:
        print("F")

    if m + f >= 80:
        print("A")

    if 65 <= m + f < 80:
        print("B")

    if 50 <= m + f < 65:
        print("C")

    if 30 <= m + f < 50:
        print("D")

    elif 30 <= m + f < 50 and r >= 50:
        print("C")

    if m + f < 30:
        print("F")

Fixed submission:
while True:
    m, f, r = [int(i) for i in input().split()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r >= 50:
            print("C")
        else:
            print("D")

    elif m + f < 30:
        print("F")

    elif m + f < 30:
        print("F")

Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = [int(i) for i in input().solit()]

    if m == f == r == -1:
        break

    if m == -1 or f == -1:
        print("F")

    if m + f >= 80:
        print("A")

    if 65 <= m + f < 80:
        print("B")

    if 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r > 50:
            print("C")
        else:
            print("D")

    if m + f < 30:
        print("F")

Fixed submission:
while True:
    m, f, r = [int(i) for i in input().split()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r >= 50:
            print("C")
        else:
            print("D")

    elif m + f < 30:
        print("F")

    elif m + f < 30:
        print("F")

Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = [int(i) for i in input().solit()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r > 50:
            print("C")
        else:
            print("D")

    elif m + f < 30:
        print("F")

Fixed submission:
while True:
    m, f, r = [int(i) for i in input().split()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r >= 50:
            print("C")
        else:
            print("D")

    elif m + f < 30:
        print("F")

    elif m + f < 30:
        print("F")

Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = [int(i) for i in input().solit()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r > 50:
            print("C")
        else:
            print("D")

    else
        print("F")

Fixed submission:
while True:
    m, f, r = [int(i) for i in input().split()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r >= 50:
            print("C")
        else:
            print("D")

    elif m + f < 30:
        print("F")

    elif m + f < 30:
        print("F")

Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = [int(i) for i in input().split()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r > 50:
            print("C")
        else:
            print("D")

    else
        print("F")

Fixed submission:
while True:
    m, f, r = [int(i) for i in input().split()]

    if m == f == r == -1:
        break

    elif m == -1 or f == -1:
        print("F")

    elif m + f >= 80:
        print("A")

    elif 65 <= m + f < 80:
        print("B")

    elif 50 <= m + f < 65:
        print("C")

    elif 30 <= m + f < 50:
        if r >= 50:
            print("C")
        else:
            print("D")

    elif m + f < 30:
        print("F")

    elif m + f < 30:
        print("F")