Problem:
 Score: 100 points    Problem Statement We have two distinct integers A and B .  Print the integer K such that |A - K| = |B - K| .  If such an integer does not exist, print IMPOSSIBLE instead.      Constraints  All values in input are integers.  0 \leq A,\ B \leq 10^9  A and B are distinct.         Input Input is given from Standard Input in the following format:  A  B       Output Print the integer K satisfying the condition.  If such an integer does not exist, print IMPOSSIBLE instead.        Sample Input 1 2 16      Sample Output 1 9  |2 - 9| = 7 and |16 - 9| = 7 , so 9 satisfies the condition.       Sample Input 2 0 3      Sample Output 2 IMPOSSIBLE  No integer satisfies the condition.       Sample Input 3 998244353 99824435      Sample Output 3 549034394

Solution (Python):
a,b = map(int, input().split())
if ((a+b)//2)*2 == a+b:
  print((a+b)//2)
else:
  print("IMPOSSIBLE")

Solution (Python):
lis=list(map(int,input().split()))
a=lis[0]+lis[1]
if a%2==0:
    print(int(a/2))
else:
    print("IMPOSSIBLE")

Solution (Python):
num = [int(i) for i in input().split()]
k = (num[0] + num[1]) / 2
if k > int(k):
  print('IMPOSSIBLE')
else:
  print(int(k))

Solution (Python):
num=input().split()
A=int(num[0])
B=int(num[1])
if A==B:
    print(0)
elif (A+B)%2==0:
    print(int((A+B)/2))
else:
    print("IMPOSSIBLE")

Solution (Python):

A = input().split()


A[0] = int(A[0])
A[1] = int(A[1])
m = 0


m = ( A[0] + A[1] ) / 2


if m.is_integer():
    print( '%d' %m)
else :
    print('IMPOSSIBLE')

Solution (Python):
import math
a,b = map(int,input().split())

if (b+a)%2==0:
    print(abs((b+a)//2))
else:
    print("IMPOSSIBLE")

Solution (Python):
import itertools

def getint(): return int(input())
def getints(): return list(map(int, input().split()))

a,b=list(sorted(getints()))


print(a+(b-a)//2 if a%2==b%2 else "IMPOSSIBLE")

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

Solution (Python):
a,b = map(int, input().split())

if (a+b)%2 == 0:
    print((a+b)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input().split())

if a % 2 != b % 2:
    print('IMPOSSIBLE')
else:
    k = (a + b) / 2
    k = int(k)
    print(k)

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

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

Solution (Python):
def test():
    a,b = map(int,input().split())


    if (abs(a)+abs(b)) % 2 == 0:
        print(int((abs(a)+abs(b))/2))
    else:
        print("IMPOSSIBLE")


if __name__ == "__main__":
    test()

Solution (Python):
a,b = map(int,input().split())
if(a==b or (a+b)%2==0):
    print((a+b)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
A,B = list(map(int,input().split()))
if A > B:
    tmp = A
    A = B
    B = tmp

if (B-A)%2 == 0:
    print(B-(B-A)//2)
else:
    print("IMPOSSIBLE")

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

Solution (Python):
ab = input()
a = int(ab.split(' ')[0])
b = int(ab.split(' ')[1])

if (a+b)%2 == 0:
   print(int((a+b)/2))
else:
   print('IMPOSSIBLE')

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

Solution (Python):
a,b = map(int,input().split())
if (a+b)%2 == 0:
  print((a+b)//2)
else:
  print('IMPOSSIBLE')

Solution (Python):
a, b = map(int,input().split(" "))

if (a+b)%2 == 0:
    print((a+b)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = [int(i) for i in input().split()]
if (a + b) / 2 % 1 == 0:
  print(int((a + b) / 2))
else:
  print("IMPOSSIBLE")

Solution (Python):
a,b=map(int,input().split())
print((a+b)//2 if (a+b)%2==0 else "IMPOSSIBLE")

Solution (Python):
import fractions;

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

ave = (a + b) // 2

if(abs(a - ave) == abs(b - ave)):
    print(ave)
else:
    print("IMPOSSIBLE")

Solution (Python):
a,b=map(int, input().split())
k=(a+b)/2
if (a+b) % 2 != 0:
    print('IMPOSSIBLE')
else:
    print(int(k))

Solution (Python):
a, b = map(int, input().split())  

if (a+b)%2==0:
    print(int((a + b) / 2))
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input().split())
if (a + b) % 2 != 0:
    print("IMPOSSIBLE")
else:
    print(int((a+b)/2))

Solution (Python):
a,b=[int(x) for x in input().split(' ')]
ans=(a+b)//2
if ans*2==a+b:
  print(ans)
else:
  print("IMPOSSIBLE")

Solution (Python):


"""
Created on Fri Aug  9 22:36:19 2019

@author: kohei
"""

N = input()

A = int(N.split()[0])
B = int(N.split()[1])

if A==B:
    print('IMPOSSIBLE')

else:
    if (A+B)%2==0:
        print(int((A+B)/2))
    
    else:
        print('IMPOSSIBLE')

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

Solution (Python):
a,b = map(int,input().split())
if abs(a - b) % 2 == 0:
    print(min(a, b) + abs(a - b) // 2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a,b=map(int,input().split())
k=(a+b)//2
if (a+b)%2==0:
    print(k)
else:
    print('IMPOSSIBLE')

Solution (Python):
A, B = map(int, input().split())
dif = abs(A - B)
print(min(A, B) + dif//2 if dif%2==0 else 'IMPOSSIBLE')

Solution (Python):
a,b = map(int,input().split())
if (a+b)%2 == 0:
	print((a+b)//2)
else:
	print("IMPOSSIBLE")

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

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

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

Solution (Python):
A, B = map(int, input().split())
if (max(A, B) - min(A, B)) % 2 == 1:
    print("IMPOSSIBLE")
else:
    print(int((A + B) / 2))

Solution (Python):
a, b = map(int, input().split())

if (max(a,b) - min(a,b)) % 2 == 1:
  print("IMPOSSIBLE")
else:
  print(int((a+b)/2))

Solution (Python):
a = list(map(int, input().split()))
b = a[0] + a[1]
if b%2 == 0:
  print(int(b/2))
else:
  print('IMPOSSIBLE')

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

Solution (Python):
"""


     = sys.stdin.readline().rstrip()
     = list(sys.stdin.readline().rstrip())


     = int(sys.stdin.readline())
     = map(int, sys.stdin.readline().split())
     = list(map(int, sys.stdin.readline().split()))
     = [list(map(int,list(sys.stdin.readline().split()))) for i in range(h)]  


a = 100
b = 0.987654321
print("{0:06d}-{1:6f}".format(a,b))
000100-0.987654


char_list = list("abcd") 
"""

import math
import sys
import itertools
import queue
from fractions import gcd

def lcm(a, b):
    return a * b // gcd(a, b)

mod = 1000000007

if __name__ == "__main__":
     A, B = map(int, sys.stdin.readline().split())
     if (A + B) % 2 == 0:
          print(int((A + B) / 2))
     else:
          print("IMPOSSIBLE")

Solution (Python):
a,b=map(int, input().split())
c = int((a+b)/2)
if a%2 ==b%2:
  print(c)
else:
  print('IMPOSSIBLE')

Solution (Python):
a, b = map(int,input().split())
print(int((a+b)/2) if a%2 == b%2 else 'IMPOSSIBLE')

Solution (Python):
a,b = map(int, input().split())
 
if (a + b) % 2 != 0:
    print("IMPOSSIBLE")
else:
    print(abs(a+b)//2)

Solution (Python):
A, B = map(int, input().split())
if (A+B) % 2 == 1:
  print("IMPOSSIBLE")
else:
  K = 1/2 * (A+B)
  print(int(K))

Solution (Python):
A, B = map(int, input().split())
if (A + B) % 2 == 1:
  print("IMPOSSIBLE")
else:
  print((A + B)//2)

Solution (Python):
a, b = map(int, input().split())
if (a+b)%2:
    print('IMPOSSIBLE')
else:
    print(int((a+b)/2))

Solution (Python):
A, B = map(int, input().split())

if A % 2 == B % 2:
    print((A + B) // 2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input().split())
if (a+b)%2 ==0:
  print((a+b)//2)
else:
  print('IMPOSSIBLE')

Solution (Python):
a, b = map(int,input().split(' '))

ans = (a+b)/2

if ans%1==0:
    print(int(ans))
else:
    print('IMPOSSIBLE')

Solution (Python):
a, b = map(int, input().split())
if (a + b) % 2 == 0:
    print(int((a+b)/2))
else:
    print('IMPOSSIBLE')

Solution (Python):
a,b=map(int,input().split())
if (a+b)%2==0:
    print((a+b)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a,b = map(int, input().split())

if (a+b) % 2 == 0:
    print(int((a+b)/2))
else:
    print("IMPOSSIBLE")

Solution (Python):
List = list(map(int,input().split()))

A = List[0]
B = List[1]

Flag = False
if (A + B) % 2 == 0:
    K = int((A + B)/2)
    Flag = True

if Flag == True:
    print(K)
else:
    print("IMPOSSIBLE")

Solution (Python):
a,b=map(int,input().split())
c=a%2
d=b%2
if (c-d)==0:
  print(int((a+b)/2))
else:
  print("IMPOSSIBLE")

Solution (Python):
A,B = map(int, input().split())
C=(A+B)/2
D=A+B
if D %2 > 0:
    print('IMPOSSIBLE')
elif D %2 == 0:
    print((A+B)//2)

Solution (Python):
A, B = map(int, input().split())
if (A+B)%2:
    result = 'IMPOSSIBLE'
else:
    result = (A+B)//2
print(result)

Solution (Python):

a,b=map(int,input().split())
print((a+b)//2 if (a+b)%2==0 else "IMPOSSIBLE")

Solution (Python):
n = input().split()
a = int(n.pop(0))
b = int(n.pop(0))

k = int((b + a) / 2)
if (b-k == k - a):
    print(k)
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input().split())
if (a-b)%2: print("IMPOSSIBLE")
else: print(min(a,b)+abs(a-b)//2)

Solution (Python):
a, b = map(int, input().split())
if (a-b)%2: print("IMPOSSIBLE")
else: print(min(a,b)+abs(a-b)//2)

Solution (Python):
a,b = map(int,input().split())

if (a+b)%2 == 0:
    print((a+b)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a,b = map(int,input().split())

if (a+b)%2 == 0:
    print((a+b)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
N = input().split()
A = int(N[0])
B = int(N[1])

if (B - A) % 2 == 0:
  print(abs(int((B - A) / 2) + A))
else:
  print("IMPOSSIBLE")

Solution (Python):
a,b = map(int, input().split())

if (a+b) % 2 == 0:
  print(int((a+b)/2))
else:
  print("IMPOSSIBLE")

Solution (Python):
A, B = map(int, input().split())
if (A-B) % 2 == 1:
    print("IMPOSSIBLE")
else: print((A+B)//2)

Solution (Python):
a,b=map(int,input().split())
if (a+b)%2==0:
  print((a+b)//2)
else:
  print('IMPOSSIBLE')

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

Solution (Python):
a, b = map(int, input().split())
print((a + b) // 2 if (a + b) % 2 == 0 else "IMPOSSIBLE")

Solution (Python):


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

if not A % 2 == B % 2:
    print("IMPOSSIBLE")
else:
    K = (A+B)/2
    print(int(K))

Solution (Python):
def solve(string):
    a, b = map(int, string.split())
    return str(abs(a + b) // 2) if ((a + b) / 2).is_integer() else "IMPOSSIBLE"


if __name__ == '__main__':
    print(solve(input()))

Solution (Python):
A, B = map(int, input().split())

if A % 2 != B % 2:
    print('IMPOSSIBLE')
else:
    print(int((A + B) / 2))

Solution (Python):
a ,b = map(int, input().split())
if (a + b) % 2 != 0:
  print('IMPOSSIBLE')
else:
  print((a + b) // 2)

Solution (Python):
A, B = map(int, input().split())

if B < A:
    A, B = B, A

if (A + B) %2 == 0:
    print ((A + B)//2)
else:
    print ('IMPOSSIBLE')

Solution (Python):
A,B=input().split(" ")
A,B=int(A),int(B)

K=A+B
if K%2==0:
  print(int(K/2))
else:
  print("IMPOSSIBLE")

Solution (Python):
A,B=map(int,input().split())

if (A+B)%2==0:
    print((A+B)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
input_data=input()
input_data=input_data.split()
A=int(input_data[0])
B=int(input_data[1])

K=(A**2-B**2)/(2*(A-B))

if int(K)==K:
    output=int(K)
else :
    output="IMPOSSIBLE"
    
print(output)

Solution (Python):
a, b = map(int, input().split())
if ((a+b)%2) == 1:
    print ('IMPOSSIBLE')
else:
    print ((a+b)//2)

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








if ((A + B) % 2) == 1:
	print("IMPOSSIBLE")
	exit(0)

K = (A + B)//2

print(K)

Solution (Python):
A,B = map(int,input().split())

if (A+B)%2 == 0:
    print((A+B)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a,b = map(int,input().split())
if (a+b)%2 == 1:
  print("IMPOSSIBLE")
else:
  print((a+b)//2)

Solution (Python):
a,b=[int(i) for i in input().split()]
if((a+b)%2):
    print('IMPOSSIBLE')
else:
    print((a+b)//2)

Solution (Python):
[a, b] = map(int, input().split())

k = (a + b) / 2
if k == int(k):
    print(int(k))
else:
    print("IMPOSSIBLE")

Solution (Python):
import sys
from collections import deque
input = sys.stdin.readline

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

if (A+B) % 2 == 0:
    print((A+B) // 2)
else:
    print('IMPOSSIBLE')

Solution (Python):
A, B = map(int, input().split())

if (A-B)%2:
    print("IMPOSSIBLE")
else:
    print(int(abs(A+B)/2))

Solution (Python):
a, b = map(int, input().split())
o = (a+b)/2
o = int(o) if o%1 == 0 else "IMPOSSIBLE"
print(o)

Solution (Python):
A,B = (int(x) for x in input(). split())

if((A+B)%2==0):
  K=(A+B)/2
  print(int(K))
else:
  K='IMPOSSIBLE'
  print(K)

Solution (Python):
temp=input().split()
A=int(temp[0])
B=int(temp[1])

m=int((A+B)/2)

if m==(A+B)/2 and m>=0:
  print(int(m))
else:
  print('IMPOSSIBLE')

Solution (Python):
a, b = tuple([int(e) for e in input().split(' ')])

if (a + b) % 2 != 0:
    print('IMPOSSIBLE')
else:
    k = int((a + b) / 2)
    if abs(a - k) == abs(b - k):
        print(k)
    else:
        print(-1 * k)

Solution (Python):
import sys
input = sys.stdin.readline

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

k2 = a+b
if k2 % 2 != 0:
    print("IMPOSSIBLE")
else:
    print(k2//2)

Solution (Python):
a,b = map(int,input().split())

if (a+b)%2 == 0:
    print(int((a+b)/2))
else:
    print("IMPOSSIBLE")

Solution (Python):
A, B = [int(i) for i in input().split()]

n = A + B
if n % 2 == 0:
    ans = n // 2
else:
    ans = 'IMPOSSIBLE'

print(ans)

Solution (Python):
a, b = map(int, input().split())
diff = abs(a - b) // 2
if max(a, b) == min(a, b) + (diff * 2):
    print(min(a, b) + diff)
else:
    print('IMPOSSIBLE')

Solution (Python):
a, b = map(int, input().split())
if (a + b) % 2 == 0:
    print(abs(a + b)//2)
else:
    print('IMPOSSIBLE')

Solution (Python):
A,B=(int(x) for x in input().split())
K=(A+B)//2
if abs(A-K) == abs(B-K):
    print(K)
else:
    print('IMPOSSIBLE')

Solution (Python):
a, b = map(int, input().split())
if (a + b) % 2 == 0:
  print(int((a+b)/2))
else:
  print('IMPOSSIBLE')

Solution (Python):
k = sum(list(map(int, input().split())))/2
if k % 1:
    print('IMPOSSIBLE')
    exit()
print(int(k))

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

Solution (Python):
a, b = map(int, input().split())
if (a-b)%2 == 0:
    print(abs(a-b)//2+min(a, b))
else:
    print("IMPOSSIBLE")

Solution (Python):
A, B = map(int,input().split())

if (A+B) % 2 > 0:
  print("IMPOSSIBLE")
else:
  print ((A+B) // 2)

Solution (Python):
a, b = map(int, input().split())

if a > b:
    c = b
    b = a
    a = c
    
if (b - a) % 2 == 1:
    print('IMPOSSIBLE')
else:
    print((b + a) // 2)

Solution (Python):
def solve():
    a, b = map(int, input().split())
    if a == b:
        return a
    elif abs(a - b) % 2 == 0:
        return int(min(a, b) + (max(a, b) - min(a, b)) / 2)
    else:
        return "IMPOSSIBLE"


if __name__ == "__main__":
    print(solve())

Solution (Python):
"""
- https://atcoder.jp/contests/abc136/tasks/abc135_a
- |A−K|=|B−K| (others:IMPOSSIBLE)
- reg: 0≤A, B≤10^9
"""

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

k = int ((a+b)/2) if (a+b) % 2 == 0 else 'IMPOSSIBLE'

print(k)

Solution (Python):
A,B = map(int,input().split())

print((A+B)//2 if (A+B)%2==0 else "IMPOSSIBLE")

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

Solution (Python):
a,b=map(int,input().split())
if (a+b)%2==0:
        print((a+b)//2)
else:
        print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input().split())
if (a + b) % 2 == 0:
    print((a + b) // 2)
else:
    print("IMPOSSIBLE")

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

Solution (Python):
input1 = input()
 
num = input1.split()
 
num1 = int(num[0])
num2 = int(num[1])
 
total = num1 + num2
 
if total%2 != 0:
	print('IMPOSSIBLE')
else:
	print(int(total/2))

Solution (Python):
a, b = map(int, input().split())
print('IMPOSSIBLE' if (b - a) % 2 else (a + b) // 2)

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

Solution (Python):


inputAB = input().split()

inputA = int(inputAB[0])
inputB = int(inputAB[1])



if inputA > inputB:
    largeNum = inputA
    smallNum = inputB
else:
    largeNum = inputB
    smallNum = inputA



if ((largeNum - smallNum) % 2) == 1:
    print("IMPOSSIBLE")
else:
    answer = smallNum + int((largeNum - smallNum) / 2)
    print(answer)

Solution (Python):


inputAB = [int(e) for e in input().split()]

inputA = int(inputAB[0])
inputB = int(inputAB[1])



if inputA > inputB:
    largeNum = inputA
    smallNum = inputB
else:
    largeNum = inputB
    smallNum = inputA



if ((largeNum - smallNum) % 2) == 1:
    print("IMPOSSIBLE")
else:
    answer = smallNum + int((largeNum - smallNum) / 2)
    print(answer)

Solution (Python):
a, b = map(int, input().split())
if (a+b)%2==0:
    print(int((a+b)/2))
else:
    print('IMPOSSIBLE')

Solution (Python):
a,b = map(int, input().split())
if (a+b)%2 == 0:
    print((a+b)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input().split())
if (a + b) % 2 != 0:
    print('IMPOSSIBLE')
else:
    print(int((a+b)/2))

Solution (Python):
A, B = map(int, input().split())
if A==B:
  K=1
  print(K)
else:
  if (A+B)%2==0:
    K=(A+B)/2
    print(int(K))
  else:
    print('IMPOSSIBLE')

Solution (Python):
num = input().split()
a = int(num[0])%2
b = int(num[1])%2
if a!=b: print('IMPOSSIBLE')
else: print((int(num[0])+int(num[1]))//2)

Solution (Python):
a, b = map(int, input().split())
if not ((a + b) % 2):
  print((a + b) // 2)
else:
  print('IMPOSSIBLE')

Solution (Python):

a, b = map(int, input().split())
 
k = a+b
if k%2==0:
  print(k//2)
else:
  print('IMPOSSIBLE')

Solution (Python):
a,b = map(int,raw_input().split())
if (a+b)%2 ==0:
    print((a+b)/2)
else: print("IMPOSSIBLE")

Solution (Python):
a,b=map(int,input().split())
if a%2!=b%2:
    print("IMPOSSIBLE")
else:
    print((a+b)//2)

Solution (Python):



b, c = map(int, raw_input().split())

if (b+c)%2 ==0:
  print((b+c)/2)
else:
  print('IMPOSSIBLE')

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

Solution (Python):
a,b=list(map(int,input().split()))
c=a+b
if (c)%2 !=0:
    print("IMPOSSIBLE")
else:
    print(c//2)

Solution (Python):
A, B = map(int, input().split())
if (A + B) % 2 == 1:
  print('IMPOSSIBLE')
else:
  print((A + B) // 2)

Solution (Python):
n = list(map(int, input().split()))

n1 = sum(n)%2
n2 = sum(n)/2

if(n1 == 0):
 print(round(n2))
else:
 print("IMPOSSIBLE")

Solution (Python):
a,b=map(int,input().split());p=abs(a-b);print(abs(max(a,b)-p//2) if p%2==0 else "IMPOSSIBLE")

Solution (Python):
a,b = map(int,input().split())
if a%2 == b%2:
  print((a+b)//2)
else:
  print("IMPOSSIBLE")

Solution (Python):



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

K = A + B
if K % 2 == 0:
    K = int((A + B) / 2)
    print(K)
else:
    print("IMPOSSIBLE")

Solution (Python):

def main():
    a,b = input().split(' ')
    a = int(a)
    b = int(b)
    c = (a + b)/2
    if int(c%1*10) >0:
        print("IMPOSSIBLE")
    else:
        print(int(c))
        
    
if __name__ == "__main__":
    main()

Solution (Python):
a, b = map(int, input().split())

if (a + b) % 2 != 0:
    print("IMPOSSIBLE")
else:
    print(int((a + b) / 2))

Solution (Python):
A,B=(int(x) for x in input().split())
C = int((A + B))

if(C%2 == 1):
    print("IMPOSSIBLE")
else:
    K = C / 2
    print(int(K))

Solution (Python):
if __name__ == '__main__':
    
    a, b = map(int,input().split())
    q, mod = divmod(a+b, 2)
    if mod == 0:
        print(q)
    else:
        print('IMPOSSIBLE')

Solution (Python):
A, B = list(map(int, input().split()))
C = A + B
if C%2 == 0:
    answer = (A + B)/2
    print(int(answer))
else:
    answer = 'IMPOSSIBLE'
    print(answer)

Solution (Python):
A,B = map(int,input().split())

if (A+B)%2 ==0:
    if A <= (A+B)//2 <= B or B <= (A+B)//2 <= A:
        print((A+B)//2)
        exit()
print("IMPOSSIBLE")

Solution (Python):
import sys




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

if __name__=='__main__':
    main()

Solution (Python):
N=list(map(int,input().split()))

a=N[0]+N[1]
if a%2==0:
    print(int(a/2))
else:
 print('IMPOSSIBLE')

Solution (Python):
a, b = map(int, input().split())
if (a+b)%2 == 0 :
    print(int((a+b)/2))
else :
    print('IMPOSSIBLE')

Solution (Python):
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)

Solution (Python):
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")

Solution (Python):
A,B=[int(x) for x in input().split()]
S=A+B
if S%2==0:
  print(S//2)
else:
  print('IMPOSSIBLE')

Solution (Python):
a,b=map(int,input().split())

if ((a+b)%2==1):
  print("IMPOSSIBLE")

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

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

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

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

if (A+B)%2 == 0:
	print((A+B)//2)
	exit()
else:
	print("IMPOSSIBLE")

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

Solution (Python):
a,b = map(int, input().split())
if (a+b)%2 == 0:
    print('{}'.format(int((a+b)/2)))
else:
    print('IMPOSSIBLE')

Solution (Python):
a,b=map(int,input().split())
if (a-b)%2==0:
  print((a+b)//2)
else:
  print('IMPOSSIBLE')

Solution (Python):
a, b = [int(e) for e in input().split()]
if (a+b)%2==0:
    print(int((a+b)/2))
else:
    print('IMPOSSIBLE')

Solution (Python):
import numpy as np

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

if (A-B)%2 == 1:
    print('IMPOSSIBLE')
else:
    print((A+B)//2)

Solution (Python):
def iread():
    return int(input())


def sread():
    return input()


def aread_int():
    tmp = input().split()
    ret = [int(i) for i in tmp]
    return ret


def aread_str():
    return input().split()


if __name__ == '__main__':
    nums = aread_int()
    if (nums[0] + nums[1]) % 2:
        print("IMPOSSIBLE")
    else:
        ans = int(((nums[0] + nums[1]) / 2))
        print(ans)

Solution (Python):


a, b = map(int, input().split())
ret = (a + b) // 2



if ret * 2 == a + b:
    print(ret)
else:
    print("IMPOSSIBLE")

Solution (Python):
a,b = map(int,input().split())
if (a+b)%2 == 0:
  print(int((a+b)/2))
else:
  print("IMPOSSIBLE")

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

ans = (a + b)/2

if ans == int(ans):
    print(int(ans))
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input("").split())
c = (a + b) / 2
print(int(c) if c.is_integer() else "IMPOSSIBLE")

Solution (Python):
A,B = map(int, input().split())
if (A+B)%2 == 1:
    print('IMPOSSIBLE')
else:
    print((A+B)//2)

Solution (Python):

def main():
    a, b = map(int,input().split())

    k = (a + b) / 2

    if k.is_integer():
        print(int(k))
    else:
        print("IMPOSSIBLE")

if __name__ == "__main__":
    main()

Solution (Python):
a, b = map(int, input().split())
if abs(a - b) % 2 != 0:
    print("IMPOSSIBLE")
else:
    print(abs(max(a,b)-abs(a-b)//2))

Solution (Python):



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


print(int((A + B) / 2) if (A + B) % 2 == 0 else 'IMPOSSIBLE')

Solution (Python):
a, b = map(int, input().split())

if (a+b)%2==1:
    print('IMPOSSIBLE')
else:
    print(int((a+b)/2))

Solution (Python):
a,b=map(int,input().split())
print('IMPOSSIBLE' if (a+b)%2 else (a+b)//2)

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

Solution (Python):
a, b = [int(i) for i in input().split()]

k = (a + b) / 2

if k.is_integer():
  print(int(k))
else:
  print("IMPOSSIBLE")

Solution (Python):
a,b = map(int,input().split())
if((a + b) % 2 == 1):
    print("IMPOSSIBLE")
else:
    ans = (a + b) // 2
    print(ans)

Solution (Python):
a,b = map(int,input().split())
if (a+b)%2 == 0:
  ans = (a+b)//2
else:
  ans = "IMPOSSIBLE"

print(ans)

Solution (Python):
A,B=map(int,input().split())

if (B-A)%2:
    print("IMPOSSIBLE")
else:
    print(int(A+(B-A)/2))

Solution (Python):
A, B = map(int, input().rstrip().split())

if (A+B) % 2 == 0:
    print(int((A+B)/2))
else:
    print("IMPOSSIBLE")

Solution (Python):
import math
a,b = list(map(int, input().split()))
ab = abs(a-b)
_max = max(a,b)
print(_max - int(ab/2) if ab % 2 == 0 else "IMPOSSIBLE")

Solution (Python):
a,b = map(int,input().split())


k = (a + b) / 2

if k.is_integer():
  print(int(k))
else:
  print('IMPOSSIBLE')

Solution (Python):
A, B = [int(_) for _ in input().split()]

M = A + B
if M % 2 == 0:
    print(M // 2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = tuple([int(_) for _ in input().split()])

k = (a + b) / 2
if k % 1 == 0.5:
    print('IMPOSSIBLE')
else:
    print(int(k))

Solution (Python):
a,b=map(int,input().split())

c=a-b

if c%2==0:
  if a<b:
    print(int(b+c//2))
  else:
    print(int(a-c//2))
else:
  print('IMPOSSIBLE')

Solution (Python):
import sys

n1, n2 = [int(i) for i in input().split()]

if (n1 % 2) != (n2 % 2):
    print("IMPOSSIBLE")
    sys.exit()

r1 = int((n1 + n2) / 2)
print(r1)

Solution (Python):
a = sum(map(int,input().split()))
print(a//2 if a%2==0 else 'IMPOSSIBLE')

Solution (Python):
def function(data):
    data = data.split(" ")
    a = int(data[0])
    b = int(data[1])
    if (a + b) % 2 == 1:
        print("IMPOSSIBLE")
    else:
        k = int((a + b) / 2)
        print(k)

if __name__ == '__main__':
  d = input()
  function(d)

Solution (Python):

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

Solution (Python):
a , b = map(int,input().split())
ans = ( a + b ) % 2

if ans != 0:
    print("IMPOSSIBLE")

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

Solution (Python):


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

Solution (Python):
A, B = map(int, input().split())
if (A + B) % 2 == 1:
    print("IMPOSSIBLE")
else:
    print(int((A + B) / 2))

Solution (Python):
A, B = map(int, input().split())

if (A+B) % 2 == 1:
    print('IMPOSSIBLE')
else:
    print(int((A+B)/2))

Solution (Python):
a,b = map(int,input().split())

s = a + b

if s%2 == 1:
    print("IMPOSSIBLE")
    exit()
print(s//2)

Solution (Python):
A, B = map(int, input().split())
if not sum([A, B]) % 2:
    print(sum([A, B]) // 2)
else:
    print('IMPOSSIBLE')

Solution (Python):
A,B = map(int, input().split())
if (A-B)%2 == 0:
  print((A+B)//2)
else:
  print('IMPOSSIBLE')

Solution (Python):
a, b = tuple(map(int, input().split(' ')))
if (a + b) % 2 == 1:
  print('IMPOSSIBLE')
else:
  print((a + b) // 2)

Solution (Python):


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

if (A + B) % 2 == 0:
    print("%d" % ((A + B) / 2))
else:
    print('IMPOSSIBLE')

Solution (Python):
a,b = map(int, input().split())
if  (b-a)%2==0:
    print(int((a+b)/2))
else:
    print("IMPOSSIBLE")

Solution (Python):
A,B=[int(i) for i in input().split()]
C=A+B
if C%2==0:
    print(int(C/2))
else:
    print("IMPOSSIBLE")

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

Solution (Python):
A, B = map(int, input().split())

if (A + B) % 2 == 1:
    print('IMPOSSIBLE')
else:
    print(int((A + B) / 2))

Solution (Python):
a, b = map(int, input().split())
c = a + b

if c % 2 == 0:
    print(int(c / 2))
else:
    print('IMPOSSIBLE')

Solution (Python):
a, b = map(int, input().split())

if a < b:
    A = a
    B = b
else:
    A = b
    B = a

if (B - A) % 2 != 0:
	print("IMPOSSIBLE")
else: 
	K = int(B - (B - A) / 2)
	print(str(K))

Solution (Python):
a, b = map(int, raw_input().split())
c = a + b
if c%2 == 0:
	print("{}".format(c/2))
else:
	print("IMPOSSIBLE")

Solution (Python):
import math
a, b = [int(i) for i in input().split()]
r = a - b if a > b else b - a
if r % 2 is 0:
  print(abs(int(r / 2)) + (a if a < b else b))
else:
  print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input().split())
print((a+b)//2) if (a+b)%2 == 0 else print('IMPOSSIBLE')

Solution (Python):
a, b = map(int, input().split())
c = (a + b) / 2
print(int(c) if c == int(c) else "IMPOSSIBLE")

Solution (Python):
A,B=map(int,input().split())
a=A
b=B
if(A>B):
    a=b
    b=A

num=a+b
if(num%2==0):
    k=num//2
    print(k)
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = [int(w) for w in input().split()]
ans = 0
if (a - b) % 2 == 1:
    print("IMPOSSIBLE")
else:
    print(abs(a + b) // 2)

Solution (Python):
A, B = map(int, input().split(' '))

if (A + B) % 2 == 1:
    print('IMPOSSIBLE')
else:
    print((A + B) // 2)

Solution (Python):
print((lambda x: x%2<1 and int(x/2) or 'IMPOSSIBLE')(sum([int(i)for i in input().split(' ')])))

Solution (Python):
a, b = map(int, input().split())
if (a+b)%2 != 0:
  print("IMPOSSIBLE")
else:
  print(int((a+b)/2))

Solution (Python):
a, b = map(int, input().split())
if (a+b)%2 != 0:
  print("IMPOSSIBLE")
else:
  print((a+b)//2)

Solution (Python):
P, Q = map(int, input().split())

A = (P + Q) / 2
if (P+Q)%2 != 0:
    print("IMPOSSIBLE")
else :
    print(round((P+Q)/2))

Solution (Python):
L,R = map(int,input().split())

ans = (L+R)/2

if ans.is_integer():
  print(int(ans))
else:
  print("IMPOSSIBLE")

Solution (Python):
a,b = map(int,input().split())
if (a+b)%2 == 0:
    print((a+b)//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a,b = map(int, input().split())
if (a+b) % 2 == 0:
  print(int((a+b)/2))
else:
  print("IMPOSSIBLE")

Solution (Python):
A,B = map(int,input().split())

s = (A+B)//2
if (A+B)%2 == 0:
    print(s)
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input().split())
if abs(a-b)%2==0:
    if a > b:
        print(int(abs(a-abs(a-b)/2)))
    else:
        print(int(abs(a+abs(a-b)/2)))
else:
    print('IMPOSSIBLE')

Solution (Python):
A, B = map(int, input().split())
K =  (A + B) // 2
if ((A + B)%2 == 0) & ((A-K)**2==(B-K)**2):
    print(K)
else:
    print("IMPOSSIBLE")

Solution (Python):
A, B = map(int, input().split())

K = (A + B ) / 2

if K % 1 == 0:
  print(int(K))
else:
  print("IMPOSSIBLE")

Solution (Python):
a,b = map(int, input().split())
if (a+b)%2 == 0:
  print(int((a+b)/2))
else:
  print('IMPOSSIBLE')

Solution (Python):
a,b=map(int,input().split())
if a%2==b%2:
  print (int((a+b)/2))
else:
  print("IMPOSSIBLE")

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

Solution (Python):
A,B = map(int,input().split())

if (A+B)%2 == 0:
  print((A+B)//2)
else:
  print("IMPOSSIBLE")