program test
  implicit none
  integer :: a,b,k
  
  read ( unit = *, fmt = * ) a, b

  if ( mod ( a + b, 2 ) == 0 ) then

     k = ( a + b ) / 2

     write ( unit = *, fmt = '(i0)')  k

  else

    write ( unit = *, fmt = '(a10)') 'IMPOSSIBLE'

  end if
  
  stop
  
end program test
