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())

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

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

Solution (Python):
"""
 // Author : snape_here - Susanta Mukherjee
 // Date : 27/07/2019

"""
from __future__ import division, print_function
import os,sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip
else:
    _str = str
    str = lambda x=b"": x if type(x) is bytes else _str(x).encode()

    


from math import ceil,floor,log,sqrt,factorial,pow,pi






  



def ii(): return int(input())
def fi(): return float(input())
def si(): return str(input())
def mi(): return map(int,input().split())
def li(): return list(mi())

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



BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def _fill(self):
        s = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
        self.seek((self.tell(), self.seek(0,2), super(FastIO, self).write(s))[0])
        return s

    def read(self):
        if self.buffer.tell():
            return self.buffer.read()
        return os.read(self._fd, os.fstat(self._fd).st_size)

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", b" "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", b"\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
input = lambda: sys.stdin.readline().rstrip(b"\r\n")



if __name__ == "__main__":
    main()

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

Solution (Python):


a, b = [int(x) for x in input().split(' ')]

if not abs(a - b) % 2:
    print(int(abs(a - b) / 2 + min(a, b)))
else:
    print('IMPOSSIBLE')

Solution (Python):
a, b = map(int, input().split())
a_even = a%2 == 0
b_even = b%2 == 0
if (a_even and b_even) or not(a_even or b_even):
    print(int((a+b)/2))
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = (int(x) for x in input().split()) 
if a == b:
    print(0)
else:
  k = (a + b) / 2
  if k.is_integer():
    print(int(k))
  else:
    print('IMPOSSIBLE')

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

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

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

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

Solution (Python):
a, b = map(int, input().split())
k = (a**2 - b**2)/(2*a - 2*b)
if((k % 1) == 0):
    print(int(k))
else:
    print('IMPOSSIBLE')

Solution (Python):



a, b= (int(i) for i in input().split())  

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

Solution (Python):
A, B = map(int, input().split())
K=(A+B)>>1
if abs(A-K) == abs(B-K):
  print(K)
else:
  print("IMPOSSIBLE")

Solution (Python):
arr =input().split(" ")
nums = int(arr[0]) + int(arr[1])
if( (nums%2) == 0):
	print(nums//2)
else:
    print("IMPOSSIBLE")

Solution (Python):
a, b = map(int, input().split())
m = (a+b)>>1
if abs(a-m) == abs(b-m):
	print(m)
else:
	print("IMPOSSIBLE")

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

Solution (Python):
def main():
  a, b = [int(_) for _ in input().split()]
  
  if (a+b) % 2 == 0:
    x = (a+b)/2
    print("{:.0f}".format(x))
    
  else:
    print("IMPOSSIBLE")
main()

Solution (Python):
s = input().split()

k = (int(s[0]) + int(s[1]))

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

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

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

k = (a + b) / 2

if (k - int(k)) == 0:
    print(int(k))
else:
    print("IMPOSSIBLE")

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

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

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

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

d = (b + c) / 2
e = (b + c) % 2

if e > 0:
	print("IMPOSSIBLE")
else:
 	print(  "%d" % d )

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

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

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

Solution (Python):
def main():
    s = input().split()
    if (int(s[0]) + int(s[1])) % 2 == 0:
        print(int((int(s[0]) + int(s[1]))/2))
    else:
        print('IMPOSSIBLE')


if __name__ == "__main__":
    main()

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 = [int(d) for d in input().split()]

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

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

Solution (Python):
k = sum(map(int, input().split())) / 2
print(int(k)) if k % 1 == 0 else print('IMPOSSIBLE')

Solution (Python):


numbers = input().split(" ")
a = int(numbers[0])
b = int(numbers[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=sum(map(int,input().split()));print("IMPOSSIBLE" if a%2 else a//2)

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

Solution (Python):

a, b = map(int, input().split())
k2 = b + a
if k2 % 2 == 0:
    print(int(k2 / 2))
else:
    print("IMPOSSIBLE")

Solution (Python):
a,b = (int(x) for x in input().split())
c = (a+b)/2
if c % 1 == 0:
    print(round(c))
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())
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 == int((a + b)/ 2):
    print(str(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(' '))

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

Solution (Python):
a, b = list(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:
  k = int((b*b - a*a)/(2*(b - a)))
  print('{}'.format(k))

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(lambda x: int(x), input().split(' '))
k = (a + b) / 2
print(int(k)) if k % 1 == 0 else print('IMPOSSIBLE')

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

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

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

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

Solution (Python):
a,b = map(int,input().split())
c = max(a,b)
d = a+b-c
k2 = c+d
if k2%2 == 0:
    print(k2//2)
else:
    print("IMPOSSIBLE")

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

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

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

Solution (Python):
a, b = map(int, input().split())
if (abs(a)+abs(b))%2 == 0:
    print(int((abs(a) + abs(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):
import sys

def input(): return sys.stdin.readline()[:-1]
def ceil(x, y=1): return int(-(-x // y))
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
def MATINT(h): return [list(map(int, input().split())) for _ in range(h)]
def MATSTR(h): return [input() for _ in range(h)]
def Yes(): print('Yes')
def No(): print('No')
def YES(): print('YES')
def NO(): print('NO')
sys.setrecursionlimit(10 ** 9)
inf = float('inf')
mod = 10 ** 9 + 7

def main():
    A, B = MAP()
    if (A + B) % 2:
        print('IMPOSSIBLE')
    else:
        print(int((A + B) / 2))

if __name__ == '__main__':
    main()

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


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

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

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

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

K = (A+B)

if K%2 != 0:
    result = "IMPOSSIBLE"
else:
    result = int(K/2)
    
print(result)

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

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

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

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

Solution (Python):
A,B = map(int, input().split())
k_2 = A + B
if k_2 % 2 == 0:
  print(k_2 // 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())
if a%2 != b%2:
    print('IMPOSSIBLE')
    exit()
else:
    k = int((a+b)/2)
    print(k)

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

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

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

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 = int(ab[0])
b = int(ab[1])
 
if (a + b) % 2 == 0:
    print((a + b) // 2)
else:
    print("IMPOSSIBLE")

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

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

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

if a == 0 and b == 0:
  print('IMPOSSIBLE')

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

Solution (Python):
a,b = map(int,input().split())
su = (a+b)/2
if abs(a-su)==abs(b-su) and ((a%2==0 and b%2==0) or (a%2==1 and b%2==1)) :print(int(su))
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=list(map(int,input().strip().split()))
print((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("IMPOSSIBLE")
elif a >= b:
    print(int(a-((a-b)/2)))
else:
    print(int(b-((b-a)/2)))

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

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

Solution (Python):

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

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

if((s[0]-s[1])%2 == 1):
  print("IMPOSSIBLE")
  exit()

if(s[0]==s[1]):
  print(0)
  exit()

if(s[0]<s[1]):
  print(int(s[0]+(s[1]-s[0])/2))
else:
  print(int(s[1]+(s[0]-s[1])/2))

Solution (Python):

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

up = (A**2-B**2)
down = (2 * ( A-B ))

if up % down == 0:
	print(int(up/down))
else:
	print("IMPOSSIBLE")

Solution (Python):
tmp = input()
fi = int(tmp.split(" ")[0])
se = int(tmp.split(" ")[1])
k = int(abs(fi + se)/2)
mod = abs(fi + se) % 2
if mod == 1:
  print("IMPOSSIBLE")
else:
  print(k)

Solution (Python):
i = list(map(int, input().split()))
a = i[0]
b = i[1]
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('IMPOSSIBLE')

else:
	K = (A+B)//2
	print(K)

Solution (Python):
A, B = map(int, input().strip().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 % 2 != b % 2:
  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(min(a,b)+ (max(a,b) - min(a, b))//2)

Solution (Python):
import sys

i = list(map(int, input().split()))

k = (i[0] + i[1]) / 2

print(int(k), end="") if int(k) == k else print("IMPOSSIBLE", end="")

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

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

mid2 = a + b

if mid2 % 2 == 0:
    mid = int(mid2 / 2)
else:
    mid = 'IMPOSSIBLE'
    
print(mid)

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

Solution (Python):
a=input().split()
num1=int(a[0])
num2=int(a[1])
k=(num1+num2)/int(2)
if  (num1+num2)%int(2)!=0:
    print("IMPOSSIBLE")
if (num1+num2)%int(2)==0:
    print(int(k))

Solution (Python):
a=input().split()
num1=int(a[0])
num2=int(a[1])
k=(num1+num2)/int(2)
if  (num1+num2)%int(2)!=0:
    print("IMPOSSIBLE")
if (num1+num2)%int(2)==0:
    print(int(k))

Solution (Python):
A,B=map(int,input().split())
def asf(A,B):
  same_num=(A+B)/2
  if same_num%1==0:
  	return int(same_num)
  else:
    return "IMPOSSIBLE"
print(asf(A,B))

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

if N[0]%2 != N[1]%2:
  print("IMPOSSIBLE")
else:
  print(int((N[0] + N[1])/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):
import sys

ab = list(map(int, input().split()))
a = ab[0]
b = ab[1]

if (a+b)%2 == 1:
  print("IMPOSSIBLE")
  sys.exit(0)

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

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

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

k = a + b

if k % 2 == 0:
    print(int(k / 2))

else:
    print('IMPOSSIBLE')

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

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

Solution (Python):
x,y = map(int,input().split())

ave = abs(x + y) / 2

if int((ave - int(ave)) * 10) == 0:
  print(int(ave))
  exit()
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(int((A+B)/2))
else:
    print('IMPOSSIBLE')

Solution (Python):
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):
import sys
def input(): return sys.stdin.readline().strip()
a,b=map(int,input().split())
k = (a+b)/2
if k == int(k):
    print(int(k))
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,input().split())
if abs(a-b) % 2 == 0:
    print(min(a,b)+(abs(a-b)//2))
else:
    print('IMPOSSIBLE')

Solution (Python):
Tmp = []
Tmp = input().rstrip().split(' ')
 
nA = int(Tmp[0])
nB = int(Tmp[1])
 
nAns = nA + nB
 
nTmp = nAns % 2
 
if nTmp == 1:
  print("IMPOSSIBLE")
else:
  nAns /= 2
  print("%d" %(nAns))

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

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

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

Solution (Python):
[A, B] = list(map(int,input().split()))
K = (A+B)//2
if 2*K == A+B:
    print(K)
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=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())
if (b-a)%2==1:
  print("IMPOSSIBLE")
else:
  m = int((a+b)/2)
  print(m)

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

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

if mod ==  0:
    print(q)
else:
    print("IMPOSSIBLE")

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=list(map(int,input().split()))
print(int(sum(a)/2) if int(sum(a)/2)==sum(a)/2 else "IMPOSSIBLE")

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

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

Solution (Python):
t=eval(input().replace(" ","+"));print((t//2,"IMPOSSIBLE")[t%2])

Solution (Python):
t=eval(input().replace(" ","+"));print(t//2 if t%2^1 else "IMPOSSIBLE")

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

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

Solution (Python):
A,B = map(int,input().split())
 
K=(A+B)/2
 
def is_integer_num(n):
    if isinstance(n, int):
        return True
    if isinstance(n, float):
        return n.is_integer()
    return False
 
if is_integer_num(K)==False:
  print("IMPOSSIBLE")
else:
  print(int(K))

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

Solution (Python):

import sys

a,b = list(map(int,input().split()))
if (a + b) % 2:
	print('IMPOSSIBLE')
	sys.exit()
print((a+b)//2)

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

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




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

Solution (Python):
a, b = map(int, input().split())
if a < b:
    tmp = b
    b = a
    a = tmp
x = (a-b)//2 + b
if abs(a-x) != abs(x-b):
    print('IMPOSSIBLE')
else:
    print(x)

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

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

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

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

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

if K % 2 == 0 :
    K = int(K/2)
else:
    K = "IMPOSSIBLE"

print(K)

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

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

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

if(N+Y)%2 == 0 :
	print((N+Y)//2)
else:
	print("IMPOSSIBLE")

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

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

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

Solution (Python):
import numpy as np
A = np.arange(1<<27,dtype=np.int32)


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

Solution (Python):
import numpy as np
A = np.zeros(1<<27,dtype=np.int32)


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

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

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


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

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 = 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 abs(A-B)%2==0:
  tmp = abs(A-B)//2
  li = [abs(A+tmp),abs(A-tmp),abs(B+tmp),abs(B-tmp)]
  li.sort()
  for i in range(3):
    if li[i]==li[i+1]:
      print(li[i])
else:
  print("IMPOSSIBLE")

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

diff = abs(A - B)

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

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

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

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

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

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

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

N, M = map(int, input().split())

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

Solution (Python):


hl = [int(i) for i in input().split()]
center = hl[0]+hl[1]

if (center % 2) == 0:
  print(int(center/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):
a, b = map(int, input().split())

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

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

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

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

k = (A+B)/2

if k.is_integer():
    print(int(k))
else:
    k = -(A+B)/2
    if k.is_integer():
        if k>0:
            print(int(k))
    else:
        print("IMPOSSIBLE")

Solution (Python):


import sys
import copy

sys.setrecursionlimit(1000000)

input = sys.stdin.readline
from collections import defaultdict



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

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

if ((a%2 == 1)and(b%2==1)):
    c = (a+b)/2
    print(int(c))

elif ((a%2==0)and(b%2==0)):
    c =(a+b)/2
    print(int(c))

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

Solution (Python):


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

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

Solution (Python):




import math
import statistics


def fact(n):
  if(n == 1):
    return 1
  else:
    return (n * fact(n - 1))


def gcd(a,b):
  if a == 0:
    return b
  elif b == 0:
    return a
  else:
    return gcd(b,a % b)


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


def qsort(seq):
  if len(seq) < 1:
      return seq
  pivot = seq[0]
  left = []
  right = []
  for x in range(1, len(seq)):
    if seq[x] <= pivot:
        left.append(seq[x])
    else:
        right.append(seq[x])
  left = qsort(left)
  right = qsort(right)
  foo = [pivot]
  return (left + foo + right)

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




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

Solution (Python):

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

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

if abs(A - k) == abs(B - k):
    print(k)
else:
    print('IMPOSSIBLE')