import math,string,itertools,fractions,heapq,collections,re,array,bisect,sys,random,time,queue,copy



sys.setrecursionlimit(10**7)

inf=10**20

mod=10**9+7

dd=[(-1,0),(0,1),(1,0),(0,-1)]

ddn=[(-1,0),(-1,1),(0,1),(1,1),(1,0),(1,-1),(0,-1),(-1,-1)]



def LI(): return [int(x) for x in sys.stdin.readline().split()]

def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]

def I(): return int(sys.stdin.readline())

def LS(): return sys.stdin.readline().split()

def S(): return eval(input())



def main():

  a,b=LI()

  

  x=max(a,b)-min(a,b)



  if x%2==1:

    return 'IMPOSSIBLE'



  return max(a,b)-x//2



# main()

print((main()))

import math,string,itertools,fractions,heapq,collections,re,array,bisect,sys,random,time,queue,copy



sys.setrecursionlimit(10**7)

inf=10**20

mod=10**9+7

dd=[(-1,0),(0,1),(1,0),(0,-1)]

ddn=[(-1,0),(-1,1),(0,1),(1,1),(1,0),(1,-1),(0,-1),(-1,-1)]



def LI(): return [int(x) for x in sys.stdin.readline().split()]

def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]

def I(): return int(sys.stdin.readline())

def LS(): return sys.stdin.readline().split()

def S(): return eval(input())



def main():

  a,b=LI()



  if a%2!=b%2:

    return 'IMPOSSIBLE'

  return (a+b)//2



# main()

print((main()))

import math, string, itertools, fractions, heapq, collections, re, array, bisect, sys, random, time, queue, copy

sys.setrecursionlimit(10**7)
inf = 10**20
mod = 10**9 + 7
dd = [(-1, 0), (0, 1), (1, 0), (0, -1)]
ddn = [(-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1), (0, -1), (-1, -1)]


def LI():
    return [int(x) for x in sys.stdin.readline().split()]


def LI_():
    return [int(x) - 1 for x in sys.stdin.readline().split()]


def I():
    return int(sys.stdin.readline())


def LS():
    return sys.stdin.readline().split()


def S():
    return eval(input())


def main():
    a, b = LI()
    x = max(a, b) - min(a, b)
    if x % 2 == 1:
        return "IMPOSSIBLE"
    return max(a, b) - x // 2


# main()
print((main()))

import math, string, itertools, fractions, heapq, collections, re, array, bisect, sys, random, time, queue, copy

sys.setrecursionlimit(10**7)
inf = 10**20
mod = 10**9 + 7
dd = [(-1, 0), (0, 1), (1, 0), (0, -1)]
ddn = [(-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1), (0, -1), (-1, -1)]


def LI():
    return [int(x) for x in sys.stdin.readline().split()]


def LI_():
    return [int(x) - 1 for x in sys.stdin.readline().split()]


def I():
    return int(sys.stdin.readline())


def LS():
    return sys.stdin.readline().split()


def S():
    return eval(input())


def main():
    a, b = LI()
    if a % 2 != b % 2:
        return "IMPOSSIBLE"
    return (a + b) // 2


# main()
print((main()))

-    x = max(a, b) - min(a, b) -    if x % 2 == 1: +    if a % 2 != b % 2: -    return max(a, b) - x // 2 +    return (a + b) // 2