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 in OCaml (4.02.3) (judge verdict: Wrong Answer)

let answer a b =
  let sa = a - b in
    if (sa mod 2) = 1 then
      "IMPOSSIBLE"
    else
      if sa < 0 then
        sa / 2 * (-1) |> string_of_int
      else
        sa / 2 |> string_of_int


let () = Scanf.scanf "%d %d" (fun a b -> print_endline (answer a b))