program main
  implicit none
  integer i,A,B
  !read(*, *) (L(i), i = 1,N)
  read(*, *)A, B
  i = (A+B)/2
  if (mod(A+B,2) == 0) then
    write(*, *) i
  else
    write(*, *) 'IMPOSSIBLE'
  end if
end program main
