ab = input().split()

A, B = int(ab[0]), int(ab[1])



if A == B:

    print((1))

elif A % 2 != B % 2:

    print("IMPOSSIBLE")

else:

    K = (A+B) // 2

    if abs(A-K) == abs(B-K):

        print(K)

    else:

        print("IMPOSSIBLE")
ab = input().split()

A, B = int(ab[0]), int(ab[1])



out = None

if A == B:

    out = None

elif A % 2 != B % 2:

    out = "IMPOSSIBLE"

else:

    K = (A+B) // 2

    if abs(A-K) == abs(B-K):

        out = K

    else:

        out = "IMPOSSIBLE"



print(out)
ab = input().split()
A, B = int(ab[0]), int(ab[1])
if A == B:
    print((1))
elif A % 2 != B % 2:
    print("IMPOSSIBLE")
else:
    K = (A + B) // 2
    if abs(A - K) == abs(B - K):
        print(K)
    else:
        print("IMPOSSIBLE")

ab = input().split()
A, B = int(ab[0]), int(ab[1])
out = None
if A == B:
    out = None
elif A % 2 != B % 2:
    out = "IMPOSSIBLE"
else:
    K = (A + B) // 2
    if abs(A - K) == abs(B - K):
        out = K
    else:
        out = "IMPOSSIBLE"
print(out)

+out = None -    print((1)) +    out = None -    print("IMPOSSIBLE") +    out = "IMPOSSIBLE" -        print(K) +        out = K -        print("IMPOSSIBLE") +        out = "IMPOSSIBLE" +print(out)