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

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

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

else:

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

if A>B:

    A,B = B,A

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

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

else:

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

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

-if (A + B) % 2 == 0: -    print(((A + B) // 2)) +if A > B: +    A, B = B, A +if (B - A) % 2 == 0: +    print(((B + A) // 2))