program tree_restoring
  implicit none
  integer :: n, a(100), d(100), l, u, i
  a = 0
  d = 0
  read(*,*) n
  read(*,*) a(1:n)
  l = minval(a(1:n))
  u = maxval(a(1:n))
  do i = 1, n
    d(a(i)) = d(a(i))+1
  end do
  do i = l+1, u
    if (d(i).lt.2) then
      write(*,'(a)') "Impossible"
      stop
    end if
  end do
  if (d(l).lt.1+mod(u,2)) then
    write(*,'(a)') "Impossible"
    stop
  end if
  write(*,'(a)') "Possible"
  stop
end program tree_restoring