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

if (A+B) % 2 == 0:

    print(((A+B)//2))

else:

    print("IMPOSSIBLE")

def LI():

    return list(map(int, input().split()))





A, B = LI()

if (A+B) % 2 == 0:

    ans = (A+B)//2

else:

    ans = "IMPOSSIBLE"

print(ans)

A, B = list(map(int, input().split()))
if (A + B) % 2 == 0:
    print(((A + B) // 2))
else:
    print("IMPOSSIBLE")

def LI():
    return list(map(int, input().split()))


A, B = LI()
if (A + B) % 2 == 0:
    ans = (A + B) // 2
else:
    ans = "IMPOSSIBLE"
print(ans)

-A, B = list(map(int, input().split())) +def LI(): +    return list(map(int, input().split())) + + +A, B = LI() -    print(((A + B) // 2)) +    ans = (A + B) // 2 -    print("IMPOSSIBLE") +    ans = "IMPOSSIBLE" +print(ans)