func readInts() -> [Int] {
    return readLine()!.split(separator: " ").map { Int($0)! }
}
    
let abk = readInts()
let a = abk[0]
let b = abk[1]
let k = abk[2]
if (a>k)&&(b>k){
  print("IMPOSSIBLE")
}else if(a<=k)&&(b<=k){
  print("IMPOSSIBLE")
}else if (a+b)%2==0{
  print(Int((a+b)/2))
}else{
  print("IMPOSSIBLE")
}
