# ABC135

def main():

    a, b = tuple(map(int, input().rstrip().split()))

    print(((a + b) // 2 if (a + b) % 2 == 0 else "IMPOSSIBLE"))





if __name__ == "__main__":

    main()
# ABC135A - Harmony

def main():

    A, B = list(map(int, input().split()))

    flg = (A + B) % 2 == 0

    print(((A + B) // 2 if flg else "IMPOSSIBLE"))





if __name__ == "__main__":

    main()
# ABC135
def main():
    a, b = tuple(map(int, input().rstrip().split()))
    print(((a + b) // 2 if (a + b) % 2 == 0 else "IMPOSSIBLE"))


if __name__ == "__main__":
    main()

# ABC135A - Harmony
def main():
    A, B = list(map(int, input().split()))
    flg = (A + B) % 2 == 0
    print(((A + B) // 2 if flg else "IMPOSSIBLE"))


if __name__ == "__main__":
    main()

-# ABC135 +# ABC135A - Harmony -    a, b = tuple(map(int, input().rstrip().split())) -    print(((a + b) // 2 if (a + b) % 2 == 0 else "IMPOSSIBLE")) +    A, B = list(map(int, input().split())) +    flg = (A + B) % 2 == 0 +    print(((A + B) // 2 if flg else "IMPOSSIBLE"))