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

c = A + B

K = c // 2



if (c % 2) != 0 :
    print("IMPOSSIBLE")

else:
    print(K)


7c06926c0b676edaa917e0c91ed2764e8cb0e464
<mask token>

<mask token>
if c % 2 != 0:
    print('IMPOSSIBLE')
else:
    print(K)

A, B = map(int, input().split())
c = A + B
K = c // 2
if c % 2 != 0:
    print('IMPOSSIBLE')
else:
    print(K)

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

c = A + B

K = c // 2



if (c % 2) != 0 :
    print("IMPOSSIBLE")

else:
    print(K)

