Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = map(int, input().split())
    if m = f = r == -1:
        break:
    elif (m == -1 or f == -1) or m+f < 30:
        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 = map(int, input().split())
    if m == -1 and f == -1 and r == -1:
        break
    elif (m == -1 or f == -1) or m+f < 30:
        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')

Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = map(int, input().split())
    if m == -1 and f == -1 and r == -1:
        break:
    elif (m == -1 or f == -1) or m+f < 30:
        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 = map(int, input().split())
    if m == -1 and f == -1 and r == -1:
        break
    elif (m == -1 or f == -1) or m+f < 30:
        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')

Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = map(int, input().split())
    if m == -1 and f == -1 and r == -1:
        break
    elif (m == -1 or f == -1) or m+f < 30:
        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 = map(int, input().split())
    if m == -1 and f == -1 and r == -1:
        break
    elif (m == -1 or f == -1) or m+f < 30:
        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')

Buggy python submission (verdict: Runtime Error):
while True:
    m, f, r = map(int, input().split())
    if m == -1 and f == -1 and r == -1:
        break
    elif (m == -1 or f == -1) or m+f < 30:
        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 = map(int, input().split())
    if m == -1 and f == -1 and r == -1:
        break
    elif (m == -1 or f == -1) or m+f < 30:
        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')