func readInts() -> [Int] {
    return readLine()!.split(separator: " ").map { Int($0)! }
}
let kotae = readInts()
let a = kotae[0]
let b = kotae[1]

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