program main
  implicit none
  integer :: a, b
  real :: k
  read *, a, b
  if (a == b) then
    print '(i0)', 0
  else if (mod(a+b, 2) == 0) then
    print '(i0)', (a+b)/2
  else
    print '(a)', 'IMPOSSIBLE'
  end if

end program
