import numpy as np

A = np.arange(1<<27,dtype=np.int32)





a,b = list(map(int,input().split()))

if (a-b) % 2 == 0:

  print(((a+b)//2))

else:

  print('IMPOSSIBLE')

import sys

read = sys.stdin.buffer.read

readline = sys.stdin.buffer.readline

readlines = sys.stdin.buffer.readlines



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



q,r = divmod(A+B,2)



if r == 1:

    print('IMPOSSIBLE')

else:

    print(q)
import numpy as np

A = np.arange(1 << 27, dtype=np.int32)
a, b = list(map(int, input().split()))
if (a - b) % 2 == 0:
    print(((a + b) // 2))
else:
    print("IMPOSSIBLE")

import sys

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
A, B = list(map(int, read().split()))
q, r = divmod(A + B, 2)
if r == 1:
    print("IMPOSSIBLE")
else:
    print(q)

-import numpy as np +import sys -A = np.arange(1 << 27, dtype=np.int32) -a, b = list(map(int, input().split())) -if (a - b) % 2 == 0: -    print(((a + b) // 2)) +read = sys.stdin.buffer.read +readline = sys.stdin.buffer.readline +readlines = sys.stdin.buffer.readlines +A, B = list(map(int, read().split())) +q, r = divmod(A + B, 2) +if r == 1: +    print("IMPOSSIBLE") -    print("IMPOSSIBLE") +    print(q)