Buggy python submission (verdict: Runtime Error):
while(1):
m,f,r = map(int,input().split())
    if (m == -1 or f == -1) or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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 == -1 and f == -1 and r == -1:
        break

Fixed submission:
while 1:
    m,f,r = map(int,input().split())
    if m == -1 and f == -1 and r == -1:
        break

    if m == -1 or f == -1 or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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")

Buggy python submission (verdict: Runtime Error):
while 1:
m,f,r = map(int,input().split())
    if (m == -1 or f == -1) or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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 == -1 and f == -1 and r == -1:
        break

Fixed submission:
while 1:
    m,f,r = map(int,input().split())
    if m == -1 and f == -1 and r == -1:
        break

    if m == -1 or f == -1 or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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")

Buggy python submission (verdict: Runtime Error):
while 1:
    m,f,r = map(int,input().split())
    if m == -1 or f == -1 or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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 == -1 and f == -1 and r == -1:
            break

Fixed submission:
while 1:
    m,f,r = map(int,input().split())
    if m == -1 and f == -1 and r == -1:
        break

    if m == -1 or f == -1 or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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")

Buggy python submission (verdict: Runtime Error):
while 1:
    m,f,r = map(int,input().split())
    if m == -1 or f == -1 or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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 == -1 and f == -1 and r == -1:
        break

Fixed submission:
while 1:
    m,f,r = map(int,input().split())
    if m == -1 and f == -1 and r == -1:
        break

    if m == -1 or f == -1 or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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")

Buggy python submission (verdict: Runtime Error):
while 1:
    m,f,r = map(int,input().split())
    if m == -1 or f == -1 or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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 == -1 and f == -1 and r == -1:
        break

Fixed submission:
while 1:
    m,f,r = map(int,input().split())
    if m == -1 and f == -1 and r == -1:
        break

    if m == -1 or f == -1 or m + f < 30:
        print("F")
    elif 80 <= m + f :
        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")