recursive function qsort(x) result(y)
  integer,intent(in) ::x(:)
  integer,allocatable::y(:)
  integer::pivot,total
  total = size(x)
  if (total <=1) then
     y = x
  else
     pivot = x(total/2)
     y = [qsort(pack(x, x .lt. pivot)), &
          pack(x, x .eq. pivot),        &
          qsort(pack(x, x .gt. pivot))]
  endif
end function qsort

program main
  interface
     recursive function qsort(x) result(y)
       integer,intent(in) ::x(:)
       integer,allocatable::y(:)
     end function qsort
  end interface

  integer::N,i,kankaku
  integer,allocatable::x(:),y(:)

  read *, N
  allocate(x(N),y(N))
  read *, x
  y = qsort(x)
  kankaku = y(2)-y(1)
  if(kankaku.eq.0) then
     print '(a)', "NO"
     return
  end if
  do i=2,N-1
     if(y(i)+kankaku.ne.y(i+1)) then
        print '(a)', "NO"
        return
     end if
  end do
  print '(a)', "YES"
end program