inf = 10 ** 18

P = 10 ** 9 + 7



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



ans = (A + B) // 2



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

    print(ans)

else:

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



K = (A + B) // 2



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

    print(K)

else:

    print('IMPOSSIBLE')
inf = 10**18
P = 10**9 + 7
A, B = list(map(int, input().split()))
ans = (A + B) // 2
if abs(A - ans) == abs(B - ans):
    print(ans)
else:
    print("IMPOSSIBLE")

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

-inf = 10**18 -P = 10**9 + 7 -ans = (A + B) // 2 -if abs(A - ans) == abs(B - ans): -    print(ans) +K = (A + B) // 2 +if abs(A - K) == abs(B - K): +    print(K)